RPM to Flow Rate Calculator
A gear pump at 1,500 RPM with 2.5 in³/rev displacement and 90% efficiency delivers about 13.1 GPM once real-world losses are applied, not the 16.2 GPM the raw arithmetic suggests. It runs the full chain: theoretical flow from displacement and RPM, your efficiency, then derating for pump type, fluid viscosity, temperature and pressure, in eight output units with a power estimate and an impeller tip-speed readout. The derating factors are planning rules of thumb, not manufacturer data.
Quick Answer
A gear pump at 1,500 RPM with 2.5 in³/rev displacement and 90% efficiency: theoretical flow is 1,500 × 2.5 ÷ 231 = 16.234 GPM; actual is 16.234 × 0.90 × 0.90 (gear-pump factor) = 13.149 GPM, a displayed volumetric efficiency of 81.0%. Add 1,500 PSI system pressure and the tool estimates shaft input power at 12.79 HP (9.54 kW). Run the same inputs above and every number matches.
Key takeaways
- The 231 in the core formula is the number of cubic inches in a US gallon, so RPM × in³/rev ÷ 231 gives GPM directly.
- The Volumetric efficiency row shows the fully derated result over the theoretical number, so with derating applied it can read lower than the efficiency you entered.
- All four derating factors are planning rules of thumb. Your pump’s published curve always wins.
- Entering pressure in bar or kPa still gives the right horsepower: the tool converts to PSI internally before the power formula runs.
How to use the RPM to flow rate calculator
Enter speed, displacement and efficiency, pick the pump type and fluid, optionally add temperature, pressure and impeller diameter, choose the output unit and calculate. Results stay hidden until you calculate.
- Set pump type, speed and displacement: the three fields that define the theoretical flow. The form is pre-filled with the worked example (gear pump, 1,500 RPM, 2.5 in³/rev). Match the displacement unit to your data plate: mL, cc, L, gal, fl oz, in³ or ft³ per revolution.
- Enter volumetric efficiency: use 100 for the pure theoretical number; healthy pumps test around 85 to 97. Anything outside 0 to 100 is rejected with an error, not a guess.
- Pick the fluid: ten fluids plus a custom option. Picking Custom Fluid reveals a viscosity field; if you leave it blank, the tool silently keeps water’s 1 cP, which flatters the result for thick fluids, so do not skip it.
- Add the optional readouts: temperature (derates outside 0 to 80°C), system pressure (unlocks the power estimate), impeller diameter (unlocks the tip-speed readout). Then choose one of eight output units and click Calculate Flow Rate.
The Reset button restores the worked-example inputs. Copy my results and Share results sit inside the results area and refuse politely until you have calculated first.
Worked example, checked against the code
Gear pump, 1,500 RPM, 2.5 in³/rev, 90% efficiency, water, 1,500 PSI, output in GPM, following the arithmetic exactly as the tool performs it.
| Scenario | Calculated flow | What the tool also says |
|---|---|---|
| Gear pump, 1,500 RPM, 2.5 in³/rev, 90%, 1,500 PSI (worked example) | 13.149 GPM | 81.0% volumetric efficiency, Good performance, 12.79 HP (9.54 kW) |
| Centrifugal, 1,750 RPM, 10 mL/rev, 85%, water, output LPM | 12.644 LPM | 72.3%, Fair performance, power N/A without pressure |
| Centrifugal on honey, 1,000 RPM, 5 L/rev, 80% | 628.730 GPM | 47.6%, Very poor, advises a positive-displacement pump |
| Centrifugal, 2,900 RPM, 50 mL/rev, 90%, oil, 10 bar, 200 mm impeller, output m³/h | 5.990 m³/h | 68.9%, tip speed 5,978.1 ft/min, 2.48 HP (1.85 kW) |
The scenario rows assume the pressure field is cleared (0 PSI) except where a pressure is stated; with the default 1,500 PSI pre-filled, the power figures come out higher.
The honey row is the one to stare at: the tool drops a centrifugal pump on 3,000 cP fluid to a 0.7 viscosity factor and flags 47.6% efficiency as very poor with a recommendation to switch pump types. That is the derating system doing its job. Every figure above was produced by the tool code itself.
Why actual flow is always less than theoretical
Three things steal flow between the arithmetic and the discharge port. Slip: clearances inside the pump let pressurized fluid leak back to the suction side, and the leak grows with pressure, which is why high-pressure systems get derated. Viscosity: thick fluids resist filling the pumping chambers, while thin hot fluids slip more; centrifugal pumps suffer more from viscosity than positive-displacement types, which the derating factors reflect. Wear: clearances open up over time, which is why the tool flags volumetric efficiency under 70% as a check-the-pump condition.
The displayed volumetric efficiency is the fully derated flow divided by the theoretical flow, times 100. Enter 90% on a gear pump and the row reads 81.0%, because the 0.90 gear-pump factor is real loss the tool refuses to hide.
The derating factors, honestly labelled
After your efficiency input, the tool applies up to four adjustment factors. Treat these as engineering rules of thumb for planning, not manufacturer data: they move the estimate in the right direction, but the pump curve for your specific pump always wins. The theoretical number is always shown alongside, so you can see exactly how much derating was applied.
| Factor | Values applied | Honest limit |
|---|---|---|
| Pump type | 0.80 (diaphragm) to 0.95 (positive displacement); gear 0.90, centrifugal 0.85 | Fixed per design; a worn pump gets the same factor. |
| Viscosity | Centrifugal: 0.7/0.8/0.9 above 100/50/10 cP. Others: 0.9/0.95 above 1,000/100 cP | Step bands, not the continuous HI 9.6.7 curves |
| Temperature | 0.9 below 0°C, 0.95 above 80°C | Only acts outside the band; blank means no derating |
| Pressure (centrifugal only) | 0.9 above 500 PSI, 0.95 above 200 PSI | Not applied to positive-displacement types |
Three more behaviours worth knowing, all verified in the code. The power estimate divides by your entered efficiency, not by the derated one: with 13.149 GPM at 1,500 PSI and 90% entered, it is 13.149 × 1,500 ÷ (1,714 × 0.90), which is the standard hydraulic horsepower formula from the Engineering ToolBox hydraulic pump reference. A blank custom viscosity keeps water’s 1 cP rather than erroring, so Custom Fluid without a number flatters thick-fluid results. And the impeller tip speed is tip speed, π × diameter × RPM ÷ 12 in ft/min, useful for wear and cavitation judgement; the fluid velocity in the pipe needs a pipe diameter and is a separate calculation, as our flow rate pressure calculator covers.
Sizing the motor
The power readout answers the natural follow-up: what motor do I need? Hydraulic power is GPM × PSI ÷ 1,714; dividing by efficiency gives the shaft input power, the same basis the Aberdeen Dynamics pump GPM guide uses. Your pressure is converted to PSI internally, so bar or kPa entries give the right horsepower, with kW beside it. Size the motor above the estimate, not at it.
What competing calculators miss
The RPM-to-flow tools ranking today mostly compute one formula and stop:
- kwcalc’s Gear Pump GPM Calculator: a correct worked GPM = displacement × RPM × efficiency ÷ 231 with a useful conversion chart, but gear pumps only, one output unit, no viscosity, temperature or pressure handling.
- CalculatorUltra’s RPM to Flow Rate: formula FR = RPM × D ÷ 231 with a clean example, but no efficiency input at all, so the number is always the theoretical maximum, which no real pump delivers.
- SavvyCalculator’s CC/REV to GPM: good FAQ on what cc/rev means, but cubic centimetres only and no derating; it tells you to consider efficiency and losses without computing them.
- calculatorshub’s GPM page: covers five GPM methods including the pump formula, but as one section of a general page with no pump-type or fluid awareness.
- The gap this page fills: ten pump types with per-design factors, ten fluids plus a custom viscosity entry, four labelled derating factors, eight output units, the always-visible theoretical number beside the derated one, a power estimate that converts your pressure to PSI first, a tip-speed readout for wear judgement, and plain-language operating-condition assessments with recommendations. If your question is gravity-driven flow rather than pumped flow, the drain flow rate calculator is the companion page; for the same RPM family in linear motion, see the RPM to distance calculator.
Frequently asked questions
The volume of fluid the pump moves in one shaft revolution, for example 2.5 cubic inches per revolution. It is fixed by the pump’s geometry and is listed on the pump’s data plate or performance curve.
One US gallon holds exactly 231 cubic inches, so RPM times in-cubed-per-rev gives cubic inches per minute, and dividing by 231 converts that to gallons per minute. The tool handles the equivalent constants for the other displacement units internally.
Actual flow divided by theoretical flow, times 100. It measures how much of each revolution’s displacement actually makes it out of the pump. New pumps typically test at 85 to 97%. Here the displayed row is the fully derated flow divided by the theoretical flow, so it can read lower than the efficiency you entered.
No. It is the speed of the impeller’s outer edge (pi times diameter times RPM divided by 12, in feet per minute), useful for wear and cavitation judgements. Fluid velocity in the pipe depends on pipe diameter and is a separate calculation.
For viscous fluids, high pressures, or when flow must stay nearly constant regardless of pressure. The tool suggests this itself when you pair a centrifugal pump with a fluid thicker than about 50 cP.
The tool keeps water’s 1 cP instead of stopping with an error. If your fluid is actually thick, enter the number: leaving it blank flatters the result. Temperature and pressure blanks are genuinely skipped, with no hidden default.
Shaft input power: hydraulic power (GPM times PSI divided by 1,714) divided by your entered efficiency. That is the power the motor must deliver, before any motor losses of its own.
The tool models displacement, speed, efficiency and derating, not suction-side hydraulics. NPSH depends on tank level, suction line losses and vapour pressure, which need a system diagram, not a pump datasheet. The recommendations flag high-RPM operation for cavitation monitoring as a prompt, not a calculation.
Add This Calculator to Your Website
Embed this calculator on your own site with the code below. Pick a size to preview it live, then copy the code.
Powered by Calculator Zilo
References
- Aberdeen Dynamics. How to Calculate GPM for a Hydraulic Pump: the standard GPM = RPM × displacement ÷ 231 derivation for hydraulic pumps. Accessed October 23, 2026.
- Engineering ToolBox. Hydraulic Oil Pump: Horsepower vs. Pressure and Volume Flow: the HP = GPM × PSI ÷ 1,714 power relation and its efficiency-adjusted form. Accessed October 23, 2026.
- FluidFlow. Viscosity Correction for Centrifugal Pumps: how pump curves rated on water are corrected for viscous service under Hydraulic Institute standard ANSI/HI 9.6.7. Accessed October 23, 2026.
- National Institute of Standards and Technology. Special Publication 811: Guide for the Use of the International System of Units (SI), conversion factors for gallon, psi, bar and related units. Accessed October 23, 2026.
- Journal of the Global Power and Propulsion Society. Centrifugal pump performance derating with non-Newtonian slurries: peer-reviewed treatment of viscosity derating factors for flow, head and efficiency per ANSI/HI 9.6.7. Accessed October 23, 2026.