Gas Orifice Size Chart and Calculator

A gas orifice is the small brass fitting with a precisely drilled hole that meters gas into a burner. The size of that hole and the gas pressure behind it set the burner's input in BTU per hour, which is why natural gas and propane (LP) appliances use different orifices. The charts below give the approximate input for every number drill size from #1 to #80 and for common metric sizes, and the orifice calculator on this page runs the same math for any pressure, heating value or altitude.

Use these numbers as a reference, not as a conversion recipe. Your appliance's rating plate and the maker's conversion kit are the authority, and orifice changes and gas conversions are work for a licensed gas installer. See Safety below.

How to Read the Orifice Size Chart

Find the size stamped on the orifice. Most burner orifices are stamped with a number drill size, such as #34 (Kozy Heat writes it "#34 (DMS)"). Some makers stamp a metric size in millimeters instead; Empire, for example, uses orifices marked 1.45, 2.25 and 2.40. The higher the drill number, the smaller the hole: a #80 is 0.0135 in. and a #1 is 0.228 in.
Read across to your gas and pressure. Each figure is the BTU per hour through one orifice at sea level, at the manifold pressure in the column heading. Manifold pressure is measured at the gas valve's outlet pressure tap with a manometer, and the right value is on the rating plate or in the manual.
Add up every orifice on the burner. A burner with front and rear orifices fires at their total. Example: an Empire DVCT36CBP9 converted to natural gas uses a #48 front orifice and a 2.40 mm rear orifice. The chart gives 16,400 + 25,300 = 41,700 BTU/hr, and Empire rates it at 40,000 BTU/hr.
Pressure matters less than you might think. Flow rises with the square root of pressure, so raising a natural gas manifold from 3.5 to 4.0 in. w.c. adds only about 7%.
Treat every figure as plus or minus 5%. Real orifices vary with the shape of the hole and the burner design. Against seven maker-rated fireplace inputs, these charts land within about 4%.
Pilot orifices are a different animal. Pilot injectors are tiny and are often marked with the maker's own code rather than a drill number (Kozy Heat marks one natural gas pilot injector "#.023NG", a decimal size). Replace a pilot injector by part number, not from this chart.

Natural Gas Orifice Size Chart (3.5 in. w.c.)

Natural gas at 3.5 in. w.c. manifold pressure, a common setting for gas fireplaces, inserts and stoves (Empire's conversion instructions, for example, set natural gas to about 3.5 in. w.c. at the valve outlet). Basis: 1,000 BTU per cubic foot, specific gravity 0.60, discharge coefficient 0.90, sea level. Values under 10,000 are rounded to the nearest 10 BTU/hr, larger values to the nearest 100.

Drill size	Diameter (in.)	Diameter (mm)	Natural gas BTU/hr at 3.5 in. w.c.
#1	0.228	5.79	147,200
#2	0.221	5.61	138,300
#3	0.213	5.41	128,500
#4	0.209	5.31	123,700
#5	0.2055	5.22	119,600
#6	0.204	5.18	117,800
#7	0.201	5.11	114,400
#8	0.199	5.05	112,100
#9	0.196	4.98	108,800
#10	0.1935	4.91	106,000
#11	0.191	4.85	103,300
#12	0.189	4.80	101,100
#13	0.185	4.70	96,900
#14	0.182	4.62	93,800
#15	0.180	4.57	91,700
#16	0.177	4.50	88,700
#17	0.173	4.39	84,700
#18	0.1695	4.31	81,300
#19	0.166	4.22	78,000
#20	0.161	4.09	73,400
#21	0.159	4.04	71,600
#22	0.157	3.99	69,800
#23	0.154	3.91	67,200
#24	0.152	3.86	65,400
#25	0.1495	3.80	63,300
#26	0.147	3.73	61,200
#27	0.144	3.66	58,700
#28	0.1405	3.57	55,900
#29	0.136	3.45	52,400
#30	0.1285	3.26	46,800
#31	0.120	3.05	40,800
#32	0.116	2.95	38,100
#33	0.113	2.87	36,200
#34	0.111	2.82	34,900
#35	0.110	2.79	34,300
#36	0.1065	2.71	32,100
#37	0.104	2.64	30,600
#38	0.1015	2.58	29,200
#39	0.0995	2.53	28,000
#40	0.098	2.49	27,200
#41	0.096	2.44	26,100
#42	0.0935	2.37	24,800
#43	0.089	2.26	22,400
#44	0.086	2.18	20,900
#45	0.082	2.08	19,000
#46	0.081	2.06	18,600
#47	0.0785	1.99	17,400
#48	0.076	1.93	16,400
#49	0.073	1.85	15,100
#50	0.070	1.78	13,900
#51	0.067	1.70	12,700
#52	0.0635	1.61	11,400
#53	0.0595	1.51	10,000
#54	0.055	1.40	8,570
#55	0.052	1.32	7,660
#56	0.0465	1.18	6,120
#57	0.043	1.09	5,240
#58	0.042	1.07	4,990
#59	0.041	1.04	4,760
#60	0.040	1.02	4,530
#61	0.039	0.99	4,310
#62	0.038	0.97	4,090
#63	0.037	0.94	3,880
#64	0.036	0.91	3,670
#65	0.035	0.89	3,470
#66	0.033	0.84	3,080
#67	0.032	0.81	2,900
#68	0.031	0.79	2,720
#69	0.0292	0.74	2,410
#70	0.028	0.71	2,220
#71	0.026	0.66	1,910
#72	0.025	0.64	1,770
#73	0.024	0.61	1,630
#74	0.0225	0.57	1,430
#75	0.021	0.53	1,250
#76	0.020	0.51	1,130
#77	0.018	0.46	920
#78	0.016	0.41	720
#79	0.0145	0.37	600
#80	0.0135	0.34	520

Propane (LP) Orifice Size Chart (10 and 11 in. w.c.)

Propane at two manifold pressures. Many gas fireplaces, inserts and stoves set propane to about 10.0 in. w.c. at the valve outlet (Empire's propane conversion instructions are one example), while the National Fuel Gas Code's propane orifice table is based on 11.0 in. w.c. Use the column that matches your rating plate or manual. Basis: 2,500 BTU per cubic foot, specific gravity 1.52, discharge coefficient 0.90, sea level.

Drill size	Diameter (in.)	Diameter (mm)	Propane BTU/hr at 10.0 in. w.c.	Propane BTU/hr at 11.0 in. w.c.
#1	0.228	5.79	390,800	409,900
#2	0.221	5.61	367,200	385,100
#3	0.213	5.41	341,100	357,700
#4	0.209	5.31	328,400	344,400
#5	0.2055	5.22	317,500	333,000
#6	0.204	5.18	312,800	328,100
#7	0.201	5.11	303,700	318,500
#8	0.199	5.05	297,700	312,200
#9	0.196	4.98	288,800	302,900
#10	0.1935	4.91	281,500	295,200
#11	0.191	4.85	274,200	287,600
#12	0.189	4.80	268,500	281,600
#13	0.185	4.70	257,300	269,800
#14	0.182	4.62	249,000	261,200
#15	0.180	4.57	243,600	255,500
#16	0.177	4.50	235,500	247,000
#17	0.173	4.39	225,000	236,000
#18	0.1695	4.31	216,000	226,500
#19	0.166	4.22	207,100	217,300
#20	0.161	4.09	194,900	204,400
#21	0.159	4.04	190,000	199,300
#22	0.157	3.99	185,300	194,300
#23	0.154	3.91	178,300	187,000
#24	0.152	3.86	173,700	182,200
#25	0.1495	3.80	168,000	176,200
#26	0.147	3.73	162,400	170,400
#27	0.144	3.66	155,900	163,500
#28	0.1405	3.57	148,400	155,600
#29	0.136	3.45	139,000	145,800
#30	0.1285	3.26	124,100	130,200
#31	0.120	3.05	108,300	113,500
#32	0.116	2.95	101,200	106,100
#33	0.113	2.87	96,000	100,700
#34	0.111	2.82	92,600	97,100
#35	0.110	2.79	91,000	95,400
#36	0.1065	2.71	85,300	89,400
#37	0.104	2.64	81,300	85,300
#38	0.1015	2.58	77,400	81,200
#39	0.0995	2.53	74,400	78,100
#40	0.098	2.49	72,200	75,700
#41	0.096	2.44	69,300	72,700
#42	0.0935	2.37	65,700	68,900
#43	0.089	2.26	59,500	62,500
#44	0.086	2.18	55,600	58,300
#45	0.082	2.08	50,500	53,000
#46	0.081	2.06	49,300	51,700
#47	0.0785	1.99	46,300	48,600
#48	0.076	1.93	43,400	45,500
#49	0.073	1.85	40,100	42,000
#50	0.070	1.78	36,800	38,600
#51	0.067	1.70	33,700	35,400
#52	0.0635	1.61	30,300	31,800
#53	0.0595	1.51	26,600	27,900
#54	0.055	1.40	22,700	23,800
#55	0.052	1.32	20,300	21,300
#56	0.0465	1.18	16,300	17,000
#57	0.043	1.09	13,900	14,600
#58	0.042	1.07	13,300	13,900
#59	0.041	1.04	12,600	13,300
#60	0.040	1.02	12,000	12,600
#61	0.039	0.99	11,400	12,000
#62	0.038	0.97	10,900	11,400
#63	0.037	0.94	10,300	10,800
#64	0.036	0.91	9,740	10,200
#65	0.035	0.89	9,210	9,660
#66	0.033	0.84	8,190	8,590
#67	0.032	0.81	7,700	8,070
#68	0.031	0.79	7,220	7,580
#69	0.0292	0.74	6,410	6,720
#70	0.028	0.71	5,890	6,180
#71	0.026	0.66	5,080	5,330
#72	0.025	0.64	4,700	4,930
#73	0.024	0.61	4,330	4,540
#74	0.0225	0.57	3,810	3,990
#75	0.021	0.53	3,320	3,480
#76	0.020	0.51	3,010	3,150
#77	0.018	0.46	2,440	2,550
#78	0.016	0.41	1,920	2,020
#79	0.0145	0.37	1,580	1,660
#80	0.0135	0.34	1,370	1,440

Metric Orifice Size Chart (mm)

Many fireplace and grill orifices are sized in millimeters, such as Empire's 1.45 mm and 1.55 mm fireplace orifices or Broilmaster's 1.05 to 2.37 mm grill orifices. Same basis as the tables above.

Orifice (mm)	Diameter (in.)	Natural gas BTU/hr at 3.5 in. w.c.	Propane BTU/hr at 10.0 in. w.c.	Propane BTU/hr at 11.0 in. w.c.
0.80	0.0315	2,810	7,460	7,820
0.85	0.0335	3,170	8,420	8,830
0.90	0.0354	3,550	9,440	9,900
0.95	0.0374	3,960	10,500	11,000
1.00	0.0394	4,390	11,700	12,200
1.05	0.0413	4,840	12,800	13,500
1.10	0.0433	5,310	14,100	14,800
1.15	0.0453	5,800	15,400	16,200
1.20	0.0472	6,320	16,800	17,600
1.25	0.0492	6,860	18,200	19,100
1.30	0.0512	7,420	19,700	20,700
1.35	0.0531	8,000	21,200	22,300
1.40	0.0551	8,600	22,800	24,000
1.45	0.0571	9,230	24,500	25,700
1.50	0.0591	9,870	26,200	27,500
1.55	0.0610	10,500	28,000	29,400
1.60	0.0630	11,200	29,800	31,300
1.65	0.0650	11,900	31,700	33,300
1.70	0.0669	12,700	33,700	35,300
1.75	0.0689	13,400	35,700	37,400
1.80	0.0709	14,200	37,800	39,600
1.85	0.0728	15,000	39,900	41,800
1.90	0.0748	15,800	42,100	44,100
1.95	0.0768	16,700	44,300	46,500
2.00	0.0787	17,600	46,600	48,900
2.05	0.0807	18,400	49,000	51,400
2.10	0.0827	19,400	51,400	53,900
2.15	0.0846	20,300	53,900	56,500
2.20	0.0866	21,200	56,400	59,100
2.25	0.0886	22,200	59,000	61,900
2.30	0.0906	23,200	61,600	64,600
2.35	0.0925	24,200	64,300	67,500
2.40	0.0945	25,300	67,100	70,400
2.45	0.0965	26,300	69,900	73,400
2.50	0.0984	27,400	72,800	76,400
2.55	0.1004	28,500	75,800	79,500
2.60	0.1024	29,700	78,800	82,600
2.65	0.1043	30,800	81,800	85,800
2.70	0.1063	32,000	84,900	89,100
2.75	0.1083	33,200	88,100	92,400
2.80	0.1102	34,400	91,400	95,800
2.85	0.1122	35,600	94,600	99,300
2.90	0.1142	36,900	98,000	102,800
2.95	0.1161	38,200	101,400	106,400
3.00	0.1181	39,500	104,900	110,000
3.05	0.1201	40,800	108,400	113,700
3.10	0.1220	42,200	112,000	117,400
3.15	0.1240	43,500	115,600	121,300
3.20	0.1260	44,900	119,300	125,100
3.25	0.1280	46,400	123,100	129,100
3.30	0.1299	47,800	126,900	133,100
3.35	0.1319	49,300	130,800	137,100
3.40	0.1339	50,700	134,700	141,300
3.45	0.1358	52,200	138,700	145,500
3.50	0.1378	53,800	142,700	149,700

Heating Values Used: 1,000 vs 2,500 BTU per Cubic Foot

The charts assume 1,000 BTU per cubic foot for natural gas and 2,500 for propane, the round numbers most charts and appliance manuals work from. Real gas varies a little:

Natural gas: gas delivered in the US averaged about 1,037 BTU per cubic foot in 2025, according to the US Energy Information Administration, so real input on typical pipeline gas can run up to about 4% above these figures. Your gas utility can tell you the heating value at your address.
Propane: propane vapor carries roughly 2,500 BTU per cubic foot at 60 °F. RegO's LP-gas manual lists 2,488 for commercial propane and the NFPA 58 property table lists 2,516, so the 2,500 basis here is within about 1% of both.
Why propane orifices are so much smaller: propane carries 2.5 times the energy per cubic foot and runs at about three times the pressure. For the same input, a propane orifice at 10 in. w.c. needs only about 38% of the hole area of a natural gas orifice at 3.5 in. w.c., or roughly 0.6 times the diameter. A 30,000 BTU/hr burner works out to a #37 or #38 orifice on natural gas and a #52 or #53 on propane.
Why the wrong orifice is dangerous: a natural gas orifice run on propane at 10 in. w.c. would pass about 2.7 times the heat it was sized for, and a propane orifice on natural gas only about 38%.

To check what an appliance is really burning, the installer can clock the gas meter with every other gas appliance off: 3,600 divided by the seconds it takes to use one cubic foot gives cubic feet per hour, and that times the heating value gives BTU per hour.

High-Altitude Orifice Sizing and Derating

These charts are for sea level. In the US, gas appliances installed above 2,000 ft are derated: input is reduced 4% for each 1,000 ft above sea level, following the American Gas Association guideline that appliance manuals cite. In Canada, the standard rating covers 0 to 4,500 ft, and above 4,500 ft the certified high-altitude rating is reduced 4% for each additional 1,000 ft (CSA B149.1).

Example: a fireplace rated 30,000 BTU/hr at sea level, installed at 5,000 ft, should run at about 30,000 × (1 − 5 × 0.04) = 24,000 BTU/hr.
The derate is done with a smaller orifice. Makers supply high-altitude orifices; Lennox's changeover and high-altitude kits for its UHCM unit heaters, for example, step from a #41 orifice up to 4,000 ft to a #44 at 4,001 to 8,000 ft and a #47 at 8,001 to 10,000 ft. Keep the manifold pressure at the rating plate value.
Check the gas, too. Some utilities at high elevation deliver gas with a lower heating value, which lowers input on its own. Carrier's furnace instructions, for example, size orifices from the heating value the local utility reports at the installed altitude. Ask your utility, then follow the appliance maker's high-altitude instructions.

When you enter an altitude above 2,000 ft, the calculator applies the 4% rule to your target input and sizes the orifice from sea-level values. That simple method leans toward a smaller orifice; the maker's high-altitude orifice size always overrides it.

Safety: Who Should Change a Gas Orifice

Orifice changes and gas conversions are work for a licensed gas installer or the dealer's service technician, not a homeowner project. The makers' conversion instructions say so: Empire's kits are to be installed by an Empire distributor or other qualified agency, and Kozy Heat warns that if its instructions are not followed exactly, a fire, explosion or carbon monoxide may result, causing property damage, personal injury or loss of life.

Convert only with the maker's kit for your exact model. A conversion is more than an orifice. Empire's fireplace kits include front and rear burner orifices, a SIT valve conversion kit and a conversion label, and the steps include resetting the air shutters; Kozy Heat's kits include burner orifices, a pilot injector, a stepper motor and labels. Find your model number first, or check the rating plate.
Do not drill, ream or substitute an orifice from a chart to change an appliance's input. Use the size the maker specifies for your model, gas and altitude.
After the work, the installer should: check every connection for leaks with soapy water, set inlet and manifold pressure with a manometer to the rating plate, verify the input (clock the meter, or check manifold pressure where there is no meter), check the main burner and pilot flames, run the safety shutdown test (on Empire's IPI fireplaces, pilot gas flow should stop within 30 seconds of turning the unit off) and fix the gas conversion label to the appliance.
If you smell gas, do not try to light any appliance. Leave the building and call your gas supplier or 911 from outside. More in our gas fireplace safety tips.

Orifices and Conversion Kits We Carry

Match the part number in your manual or on your model's parts list before ordering. A kit or orifice made for one model is not a universal part, even when the stamped size matches.

What you are doing	What it takes	Part to check	Parts we carry
Converting a fireplace, insert or stove from natural gas to propane, or back	The maker's conversion kit for your exact model: burner orifices, pilot injector or pilot orifice, valve conversion parts and a conversion label	Conversion kit by model number (find it in the manual or on the rating plate)	Empire 31054, NG to LP (DVP20MS, DVP20CC) Empire 32877, LP to NG (DVCP36BP, DVCC36BP) Empire BVCK36P, NG to LP (BVD34FP, BVD36FP) Empire 40513, LP to NG (DVLL36BP) Majestic LPK-ECHEL48 (LP) Superior GCKSLLXNP, NG to LP Superior GCKSIT3035MNP, NG to LP (millivolt) HHT 40LCKN (NG) Heatilator and Heat N Glo LPK-4836 (LP) Astria Scorpio and Superior F2183 (LP) All gas valves and conversion kits
Replacing a damaged, cross-threaded or lost main burner orifice (same gas)	A genuine orifice with the same stamped size and thread as the original	Burner orifice by part number (front and rear orifices are often different sizes)	Empire P208, 1.45 mm Empire P258, 1.55 mm Empire P185, #52 Empire P182, #55 Empire P243, #36 (VFLB48FP) Empire P193, #64 Buck PO 44MVORF, NG rear Buck PO 54MVORF, LP rear All Empire parts
Pilot injector damaged, or the pilot needs the other gas	Pilot orifice (injector) for the gas type; pilot injectors are marked with the maker's own codes	Pilot orifice by part number	SIT 0.977.166 pilot orifice, NG SIT 0.977.167 pilot orifice, LP HHT 446-517 pilot orifice, LP All HHT parts
Converting a gas grill	The grill maker's conversion kit, or the matching orifices	Grill conversion kit or burner orifice	Broilmaster BCK1000, LP to NG (P3, P4, D3, D4) Broilmaster BCK1001, NG to LP (P3, P4, D3, D4) Broilmaster B056647, NG orifice (P and D series) Broilmaster BR0047, 1.30 mm orifice MHP GGNAT-KIT, LP to NG (WNK, TJK) MHP GGLP-KIT, NG to LP (WNK, TJK) All Broilmaster parts
Converting a furnace	The furnace maker's LP or natural gas kit for that model	Furnace conversion kit	Lennox 25W20, NG to LP Lennox 44W49, LP to NG Carrier KGAPN3901ALL, LP to NG Rheem FP-02 (LP)

Find parts for your exact appliance: gas valves and conversion kits · Empire fireplace parts · HHT (Heat N Glo, Heatilator, Quadra-Fire) parts · Broilmaster grill parts.

Gas Orifice FAQ

What size orifice do I need for my BTU rating?

Divide the rated input by the number of orifices, then find the drill size closest to that figure without going over, in the column for your gas and pressure. For 30,000 BTU/hr on one orifice, that is a #38 (about 29,200 BTU/hr) on natural gas at 3.5 in. w.c., or a #53 (about 26,600) to #52 (about 30,300) on propane at 10 in. w.c. On a fireplace, the orifice the maker lists for your model always wins.

What is the difference between a natural gas and a propane orifice?

The hole size. For the same input, a propane orifice is roughly 0.6 times the diameter of the natural gas one, because propane carries more energy per cubic foot and runs at higher pressure. The fittings can look alike, so read the stamped size.

Can I convert my gas fireplace by changing the orifice?

No. A conversion also changes the valve's regulated pressure (about 3.5 in. w.c. on natural gas vs 10.0 on propane for Empire's fireplaces, done with a SIT valve conversion kit), often the pilot injector, and sometimes the air shutter setting. Use the maker's kit for your model and have a qualified installer do the work.

How do I adjust orifice size for high altitude?

In the US, reduce the sea-level input 4% for every 1,000 ft above sea level once you are above 2,000 ft, and use the smaller orifice the maker specifies for that elevation. At 6,000 ft that is a 24% reduction. Also ask your utility for the heating value of your gas, because lower-BTU gas at altitude already reduces input.

Are orifice sizes the same as drill sizes?

Usually. Most orifices are stamped with a number drill size (#1 to #80) or a metric diameter in millimeters, and the hole matches that drill. The drill diameters in these charts follow the ASME B94.11M twist drill standard. Pilot injectors are the exception; many carry the maker's own code.

How These Numbers Were Calculated

Every value comes from the standard orifice flow equation: Q = 1658.5 × Cd × A × √(P ÷ G), where Q is cubic feet per hour, A is the hole area in square inches (π × d² ÷ 4), P is the manifold pressure in inches of water column, G is the gas specific gravity (air = 1) and Cd is the discharge coefficient. BTU per hour = Q × heating value. We used Cd 0.90 and specific gravity 0.60 for natural gas, values that reproduce the National Fuel Gas Code (NFPA 54) natural gas orifice table, and specific gravity 1.52 for propane.

We checked the results three ways. Against the NFPA 54 natural gas values at 3.5 in. w.c., 62 of 63 drill sizes agree within 5% and 56 within 1%; the one miss is a misprinted row in the reprint we used. Against the NFPA 54 propane table at 11 in. w.c., reprinted in RegO's LP-Gas Serviceman's Manual, our propane values run 2% to 3% lower on the same heating value, on every row but one misprinted one. Against seven maker-rated fireplace inputs from Empire and Kozy Heat conversion instructions, the computed input lands within about 4% of the rating every time (the widest gap is 4.1%, Empire's DVCT36 on natural gas).

Gas orifice size calculator

Find the drill size for a BTU rating, or the BTU rating of an orifice you have, for natural gas or propane. Results are estimates; the rating plate and the maker's conversion kit decide.

The calculator needs JavaScript. The gas orifice size chart on this page lists the same values for every drill size.

What do you want to find?

Orifice size from BTU/hr BTU/hr from orifice size

Gas type

Natural gas Propane (LP)

Rated input (BTU/hr, sea level)

From the rating plate. If the plate already shows a high-altitude input, enter that and leave altitude at 0.

Orifice size

The number or millimeter size stamped on the orifice. Choose "Other diameter" for a measured hole.

Number of orifices

Orifices sharing the input. Many fireplace burners use two different sizes; the maker's sizes always win.

Manifold pressure (in. w.c.)

Typical: natural gas 3.5, propane 10.0 (some ratings use 11.0). Use your rating plate value.

Altitude (ft above sea level)

Above 2,000 ft, US practice reduces input 4% per 1,000 ft above sea level.

Advanced: gas properties

Heating value (BTU per cu ft)

Specific gravity (air = 1)

Discharge coefficient

Estimates only. These figures come from the standard orifice equation (discharge coefficient 0.90) and are accurate to roughly plus or minus 5%. Real orifices, burners and gas vary. Your appliance's rating plate and the maker's conversion kit always take priority, and orifice changes or gas conversions must be done by a qualified gas installer.