lb/hr to cc/min Converter: Fuel Injector Flow Rate

Use this converter to change fuel injector flow rate between lb/hr, cc/min and g/min, using the fuel specific gravity you choose rather than one fixed multiplier.

How the lb/hr to cc/min conversion works

lb/hr measures mass. cc/min measures volume. You cannot move between them without knowing how heavy the fuel is, which is what specific gravity tells you.

One avoirdupois pound is 453.59237 grams, exactly, by definition. Spread over an hour that is 7.55987 grams per minute. Divide that by the fuel’s specific gravity and you have the volume:

cc/min = lb/hr × 453.59237 ÷ (60 × specific gravity)
lb/hr  = cc/min × 60 × specific gravity ÷ 453.59237

Every part of that is fixed except the specific gravity. That is the number your fuel choice changes, and it is the number most converters never show you.

Why 10.5 is not a constant

Injector manufacturers publish the shortcut. RC Engineering states it plainly: to convert cc/min to lb/hr, divide by 10.5. The factor is real and widely used, and for pump petrol it is close enough for most sizing work.

But 10.5 is a specific gravity wearing a disguise. Work backwards through the formula and the specific gravity sitting behind that shortcut is 0.71999. Anyone using the factor is assuming that number, whether or not they know it.

That matters as soon as the fuel is not the fuel the shortcut assumes. A converter that prints a single factor, with no specific gravity next to it, cannot tell you when it stops applying. The page ranking first for this conversion today gives the 10.5 formula and states no specific gravity anywhere on it.

The one conversion that does not depend on fuel

lb/hr and g/min both measure mass, so converting between them needs no specific gravity at all. One lb/hr is 7.55987 g/min for petrol, E85, methanol, diesel or water.

This is worth knowing because injector data sheets and flow-bench reports often quote g/min. If your two numbers are both mass units, fuel type is irrelevant and any disagreement is a measurement problem, not a conversion problem. The moment cc/min enters, specific gravity is back.

E85 changes the answer

Rockett Brand publishes a specific gravity table for its E85, measured against temperature. At 60F the specific gravity is 0.7887, which gives a conversion factor of 9.585 rather than 10.5.

On a 60 lb/hr injector that is the difference between 630.00 cc/min on the shortcut and 575.11 cc/min on E85, a gap of 54.89 cc/min. If you are shopping against a cc/min target, that is enough to point you at the wrong injector.

The same table shows why a single number is the wrong shape for this. Specific gravity moves with temperature: 0.7970 at 40F down to 0.7725 at 99F, a spread of 3.17% in the resulting factor across the range the table covers. E85 is also a blend that varies by grade and season, so the table describes one manufacturer’s product rather than whatever is in the pump.

The practical answer is to measure or look up the specific gravity of the fuel you are actually running, then enter it above. A hydrometer and a thermometer are what the Rockett table is designed to be read against.

Other conversions on the same page

A US liquid gallon is 231 cubic inches exactly, and an inch is 2.54 centimetres exactly, so a gallon is 3785.411784 cc. One gallon per hour is therefore 63.09 cc/min. That conversion is pure geometry and does not involve fuel density at all.

FAQ

Is 10.5 wrong?

No, but it is narrower than it looks. It is the factor for a specific gravity of 0.71999, which is a reasonable stand-in for pump petrol. Used on E85, methanol or diesel it will read high, because those fuels are denser.

Which specific gravity should I enter?

Whatever your fuel supplier or test sheet states, at the temperature it was measured. If you have no figure, the 10.5 convention is the usual default for petrol, and this page shows you the assumption it carries.

Why does my injector data sheet disagree with this result?

Injector ratings are published at a base fuel pressure, and flow changes when the pressure does. Convert the units first, then adjust for pressure with the Fuel Pressure Flow Calculator.

Does temperature really matter?

For unit conversion it matters through specific gravity only, and the effect is small but not nothing: the E85 table above moves 3.17% across 40F to 99F. For sizing decisions with a safety margin that is usually noise. For matching a target cc/min exactly it is not.

Where to go next

If you are choosing an injector rather than converting one, use the Fuel Injector Size Calculator to get a required flow rate from horsepower, BSFC and duty cycle. To check whether the injectors you already have still have headroom, use the Fuel Injector Duty Cycle Calculator. To size the rest of the fuel system, use the Fuel Pump Size Calculator, or browse the engine calculators hub.

Sources and assumptions

This converter assumes the specific gravity shown in the result panel and applies no pressure, temperature or voltage correction. Injector ratings are published at a base fuel pressure, so convert units here and adjust for pressure separately. Results are estimates for planning and comparison, not a substitute for a flow-bench test of your actual injectors.

• NIST Handbook 133 (2026), Appendix E, General Tables of Units of Measurement for the exact pound, gallon and inch definitions
• Rockett Brand Tech Bulletin 112, E85 Temperature / Specific Gravity Table for E85 specific gravity against temperature
• RC Fuel Injection technical page for the 10.5 industry convention