Rpm to Flow Rate Calculator

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RPM to Flow Rate Calculator

Cubic Inches (CID) Cubic Centimeters (CC)

Calculation Results

Theoretical Flow:

Actual Flow (with Efficiency):

function calculateFlowRate() { var disp = parseFloat(document.getElementById('displacement').value); var unit = document.getElementById('dispUnit').value; var rpm = parseFloat(document.getElementById('rpm').value); var eff = parseFloat(document.getElementById('efficiency').value); var resultArea = document.getElementById('resultArea'); var theoFlowSpan = document.getElementById('theoFlow'); var actualFlowSpan = document.getElementById('actualFlow'); if (isNaN(disp) || isNaN(rpm) || isNaN(eff) || disp <= 0 || rpm <= 0) { alert('Please enter valid positive numbers for all fields.'); return; } var effDecimal = eff / 100; var gpm, lpm, theoreticalGPM, theoreticalLPM; if (unit === 'cid') { // Formula: (CID * RPM) / 231 = GPM theoreticalGPM = (disp * rpm) / 231; gpm = theoreticalGPM * effDecimal; theoreticalLPM = theoreticalGPM * 3.78541; lpm = gpm * 3.78541; } else { // Formula: (CC * RPM) / 1000 = LPM theoreticalLPM = (disp * rpm) / 1000; lpm = theoreticalLPM * effDecimal; theoreticalGPM = theoreticalLPM / 3.78541; gpm = lpm / 3.78541; } theoFlowSpan.innerHTML = theoreticalGPM.toFixed(2) + " GPM (" + theoreticalLPM.toFixed(2) + " LPM)"; actualFlowSpan.innerHTML = gpm.toFixed(2) + " GPM (" + lpm.toFixed(2) + " LPM)"; resultArea.style.display = 'block'; }

How to Convert RPM to Flow Rate

In hydraulic systems and fluid power engineering, determining the output flow of a pump based on its rotational speed (RPM) and displacement is a fundamental task. This calculator helps you quickly bridge the gap between mechanical rotation and fluid movement.

The Physics of Pump Flow

Flow rate is essentially the volume of fluid moved over a specific period. For positive displacement pumps, every single revolution of the shaft moves a fixed volume of fluid. By multiplying this displacement by the speed of rotation, you get the theoretical flow rate.

Standard Formulas

  • Imperial System: Flow (GPM) = (Displacement in CID × RPM) / 231
  • Metric System: Flow (LPM) = (Displacement in CC × RPM) / 1000

The constant 231 is used in the Imperial formula because there are exactly 231 cubic inches in one US gallon. In the metric system, 1000 cubic centimeters equal 1 liter, making the math much more straightforward.

Understanding Volumetric Efficiency

In the real world, pumps are not 100% efficient. Due to internal leakages (often called "slip"), friction, and fluid compressibility, the actual flow rate is always slightly lower than the theoretical flow. This is where Volumetric Efficiency comes in. Most high-quality hydraulic pumps operate between 85% and 95% efficiency. Our calculator allows you to input this percentage to get an accurate "Actual Flow" reading.

Example Calculation

Imagine you have a hydraulic pump with a displacement of 3.0 Cubic Inches (CID) running at a motor speed of 1,800 RPM with an efficiency of 90%.

  1. Theoretical Flow: (3.0 × 1,800) / 231 = 23.37 GPM
  2. Actual Flow: 23.37 × 0.90 = 21.03 GPM

Why is this important?

Knowing the precise flow rate is critical for sizing hydraulic hoses, valves, and actuators. If your flow rate is too high for your plumbing, you will experience excessive heat generation and pressure drops. Conversely, if the RPM or pump displacement is too low, your hydraulic cylinders or motors will move slower than required for the application.

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