A float switch changes electrical state as a buoyant float moves with liquid level. It can provide a level signal for pump control or an alarm, using a suitable control circuit. This calculator finds the volume between two levels in a cylindrical sump and the refill rate for entered inflows; it does not calculate complete pump cycles.
Float Switch Interactive Calculator
Compare differential volume and refill rates for three steady inflows. Pump-down time and pump discharge flow are not included.
Equation Used
Differential volume is the liquid volume between the selected on and off levels. Dividing inflow by this volume gives the reciprocal of refill time, not actual pump starts per hour. A complete cycle also includes pump-down time, which this tool does not calculate.
- Cylindrical sump with vertical sides.
- Float differential is the vertical distance between pump-on and pump-off levels.
- The displayed rates are Q/V refill comparisons; they omit pump-down time and do not predict motor duty.
- Inflow is treated as steady for each demand case.
Liquid level moves the float
Buoyancy causes a suitable float to follow the liquid surface. A tethered float may tilt to operate a switch, while a vertical magnetic float can travel along a stem. In the illustrated reed-switch arrangement, a magnet in the float operates contacts sealed inside the stem.
The switching behavior depends on the product: normally open or closed contacts, operating position and differential are not universal. A level switch supplies a switching state rather than a continuous measurement of the whole tank level.
Float material, liquid density, deposits and freedom of movement affect operation. Contact ratings must match the intended electrical load or control interface; the simple circuit drawing is a principle diagram, not a pump-wiring design.
Level control and alarms
Float switches can provide high- or low-level signals in tanks and sumps. A controller may use separate levels for pump operation and alarms, depending on the system.
Select the float and control circuit for the liquid, temperature, environment and electrical load. The calculator has no liquid-density, contact-rating or pump-performance input and cannot make that selection.
Differential volume and refill rate
For sump diameter D and vertical level difference h, both in millimetres, V = π(D/1000)²(h/1000) × 1000/4 gives litres. For steady inflow Q in litres per hour, the displayed refill rate is Q/V per hour and refill time is V/Q hours.
A pump-down cycle also needs time to empty that volume. With a constant discharge P greater than constant inflow Q, ideal cycle time would be V/Q + V/(P − Q), assuming the same two switching levels and no other flows. This calculator has no P input, so it does not evaluate that full cycle.
Q/V approaches actual cycles per hour only when pump-down time is negligible. It is not a motor-start limit, service-life estimate or instruction for setting the float tether.
Example: a cylindrical sump
Enter diameter 600 mm and a 100 mm level difference. The differential volume is 28.274 L. For inflows of 1,200, 600 and 150 L/h, the displayed refill rates are 42.44, 21.22 and 5.31 per hour.
At 600 L/h, the sump takes about 2.83 minutes to refill that volume. A pump’s running time would then add to the complete cycle. The 21.22 per-hour result is therefore not a prediction of actual pump starts.
Doubling level difference doubles the volume and halves these refill rates. Whether those switching levels are suitable depends on available space, the pump and the switch instructions.
Selecting a level sensor
A float provides a mechanical response to liquid level. Other sensing methods use conductivity, capacitance, light or sound. Their suitability depends on the fluid and installation rather than a universal cost or reliability ranking.
Use the exact pump and switch instructions for operating levels, electrical interfaces and permitted cycling. This page gives no universal tether length, clearance, contact current, suppression circuit or replacement interval.
Frequently asked questions
Are the displayed rates actual pump starts per hour?
No. They are inflow divided by differential volume. Pump-down time is missing, so the results describe refill rates.
Why does diameter have a strong effect?
The cross-sectional area of a cylindrical sump is proportional to diameter squared. At the same level difference, doubling diameter gives four times the differential volume.
Does Float Diff. mean tether length?
No. It is the vertical liquid-level difference between the selected on and off levels. Tether geometry is product-specific.
Can the tool check whether the pump keeps up?
No. There is no pump discharge-flow input or performance curve.
Can a float switch drive a motor directly?
Only if the exact switch is rated for that motor load and the installation follows its instructions. A level-signal switch may require a suitable relay or controller.
Does the diagram specify a universal stem clearance?
No. The gap and geometry are illustrative. Use the actual product’s fluid, mounting and movement requirements.
Manufacturer references
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