Quick pump duty point: flow + total head from static lift, friction, and minor losses.
Includes kPa ↔ m conversions and indicative power check. Tender-stage only. Final selection needs pump curves / NPSH.
Calculations per CIBSE Guide G / BS EN ISO 9906 (pump testing). For design guidance only. Final selection requires manufacturer pump curves and NPSH datasheet.
About this pump duty calculator
This free pump duty calculator works out the duty point, flow and total head, for a building-services pump, then gives an indicative power and an optional NPSH suction check. It is aimed at UK building-services engineers and contractors sizing circulators and end-suction pumps at tender stage. Enter flow in L/s, L/min or m³/h and either build the head up from static lift, friction and minor losses (with a design margin) or enter the total head directly. Results report flow, total head, pressure in kPa and input power, with a full head build-up breakdown. It is a sanity check only, final selection still needs manufacturer pump curves and an NPSH datasheet.
How the duty point is calculated
Total head is the sum of static lift, friction loss and minor (valve and fitting) losses, plus a design margin (typically 5–15 %). Friction and minor losses can be entered in kPa or metres head, converted using Δp = ρ·g·h ÷ 1000. Hydraulic power is Ph = ρ·g·Q·H and input power is Ph ÷ η. The optional suction check computes NPSHa = (Patm − Pv) ÷ (ρ·g) + Zs − Hfs and flags a cavitation risk where NPSHa falls below NPSHr plus a 0.5 m margin. Calculations follow CIBSE Guide G and BS EN ISO 9906 (pump testing).
Frequently asked questions
Can I enter total head directly instead of building it up?
Yes. Switch the head input mode to “Enter total head directly” and type the total head in metres; the static, friction, minor and margin fields are then hidden and the entered value is used as the duty head.
What efficiency and fluid density should I use?
Density defaults to 998 kg/m³ (water near 20 °C) and accepts 800–1200 to cover glycol and brine; gravity defaults to 9.81 m/s². Pump efficiency defaults to 65 %, with the field noting typical ranges of 65–75 % for a VSD circulator and 70–80 % for an end-suction pump.
Does the NPSH check replace a proper selection?
No. The NPSHa figure is an indicative suction check; you supply the pump NPSHr from its curve to enable the margin flag. As the calculator notes, it is for design guidance only and final selection requires manufacturer pump curves and an NPSH datasheet.