Nozzle conversion: stamped GPH to actual, both directions

For the techs. Nozzles are rated at 100 PSI and almost nothing runs at 100 PSI, so every substitution is a square root problem at the worst possible time to do algebra. Convert either direction here, or print the chart for the visor. Actual flow equals stamped GPH times the square root of pressure over 100.

Stamped to actual Required rate to stamp

Stamped nozzle GPH Pump pressure (PSI)

Enter a nozzle size or a required firing rate. Pressure defaults to 140 PSI.

The chart

Stamped	100	110	125	140	145	150	160	175	200
0.40	0.40	0.42	0.45	0.47	0.48	0.49	0.51	0.53	0.57
0.50	0.50	0.52	0.56	0.59	0.60	0.61	0.63	0.66	0.71
0.55	0.55	0.58	0.61	0.65	0.66	0.67	0.70	0.73	0.78
0.60	0.60	0.63	0.67	0.71	0.72	0.73	0.76	0.79	0.85
0.65	0.65	0.68	0.73	0.77	0.78	0.80	0.82	0.86	0.92
0.75	0.75	0.79	0.84	0.89	0.90	0.92	0.95	0.99	1.06
0.85	0.85	0.89	0.95	1.01	1.02	1.04	1.08	1.12	1.20
0.90	0.90	0.94	1.01	1.06	1.08	1.10	1.14	1.19	1.27
1.00	1.00	1.05	1.12	1.18	1.20	1.22	1.26	1.32	1.41
1.10	1.10	1.15	1.23	1.30	1.32	1.35	1.39	1.46	1.56
1.20	1.20	1.26	1.34	1.42	1.44	1.47	1.52	1.59	1.70
1.25	1.25	1.31	1.40	1.48	1.51	1.53	1.58	1.65	1.77
1.35	1.35	1.42	1.51	1.60	1.63	1.65	1.71	1.79	1.91
1.50	1.50	1.57	1.68	1.77	1.81	1.84	1.90	1.98	2.12
1.65	1.65	1.73	1.84	1.95	1.99	2.02	2.09	2.18	2.33
1.75	1.75	1.84	1.96	2.07	2.11	2.14	2.21	2.32	2.47
2.00	2.00	2.10	2.24	2.37	2.41	2.45	2.53	2.65	2.83
2.25	2.25	2.36	2.52	2.66	2.71	2.76	2.85	2.98	3.18
2.50	2.50	2.62	2.80	2.96	3.01	3.06	3.16	3.31	3.54
3.00	3.00	3.15	3.35	3.55	3.61	3.67	3.79	3.97	4.24

Actual GPH = stamped GPH × √(PSI ÷ 100). Columns are pump pressure in PSI. Computed values; in-spec new nozzles track this closely, worn nozzles run high.

Notes for the field

The reverse lookup divides instead of multiplying: required rate ÷ √(PSI ÷ 100) gives the equivalent 100 PSI stamp, then pick the closest size the line actually makes.
Firing rate is the appliance's spec, not the nozzle's. Match the input rating on the boiler or furnace plate, and respect the burner manufacturer's nozzle and pressure tables where published; this page is arithmetic, not an application chart.
Spray angle and pattern still rule. The math only converts flow. Angle and pattern come from the equipment spec or what the combustion chamber proved it wants.
Homeowner reading this? Nozzle selection and pump pressure are licensed-tech work. The page you want is the burner fuel use calculator, which turns your existing nozzle and hours into gallons.

Common questions

Why are nozzles rated at 100 PSI if burners run higher?

The 100 PSI rating is the industry test standard, old enough to predate flame retention burners. Modern equipment runs 140 to 150 PSI and up for better atomization, so the stamped number is a reference point, not the delivered flow. That gap is exactly what this chart converts.

I need 1.10 GPH at 140 PSI. What do I install?

Divide: 1.10 ÷ √1.4 is about 0.93, so a 0.90 stamp lands at 1.06 actual and a 1.00 stamp at 1.18. Pick per the appliance input target and the burner maker's table; the reverse mode above does this arithmetic and shows both neighbors.

Does pressure change the spray angle too?

Higher pressure mainly improves atomization and raises flow; the nominal angle and pattern keep their designation. Severely off-spec pressure can distort the pattern, which is one more reason to set pressure to the burner spec with a gauge rather than by ear.

How accurate is the square root rule?

Within a few percent for a clean, in-spec pressure-atomizing nozzle across the residential pressure range, which is why the published manufacturer tables follow the same curve. Wear, partial plugging, and viscosity swings move real flow off the math, always upward for wear.

Burner fuel use calculator (hours to gallons, homeowner version)
No gauge, no stick: tracking oil
All tank charts

Nozzle conversion: stamped GPH to actual, both directions

The chart

Notes for the field

Common questions

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