Room Heat Loss
Room-by-room heat load following BS EN 12831-1: walls, windows, floor, roof, ventilation and design temperatures, in watts and BTU/h. The outdoor design temperature for your postcode is filled in for you.
For heating engineers and installers
Every heating calculation your job needs, offline on your phone.
Heat loss, radiator sizing, heat pump and boiler calculations, cylinders and gas. All running on your Android phone, no account and no signal needed.
From room heat loss to pipe diameter
Radiator Sizing
Corrected radiator output at your actual flow and return temperatures using the EN 442 ΔT method. See whether an existing radiator is large enough for a heat pump at 45 °C or 50 °C, or for the 55 °C Part L limit, and the exact size you need to make up any shortfall.
Heat Pump and Boiler Sizing
Total design heat load plus hot-water allowance for heat pump selection, with the flow rate the system requires. System, heat-only and combi boiler sizing from the building heat loss, plus the kW a combi needs for the number of showers and taps you want to run at once.
Hot Water and Cylinders
Cylinder volume and reheat time for the household. Coil output at your primary flow temperature, including heat pump cylinders. Unvented cylinder expansion vessel size and pre-charge. Sealed system heating vessel sizing from system volume, fill pressure and maximum temperature.
Gas Rate and Pipe Sizing
Convert a metric or imperial meter reading to kW heat input and compare it against the appliance data plate. Size gas pipe runs by length, load and allowable pressure drop.
Projects and PDF Reports
Save all calculations for a job into one project. Export a combined PDF report to share with the customer or keep on file. Flowtemp Pro adds your own logo to the exported report.
Three things that matter on site
Fully offline
All calculations run on the device. There is no server call, no account and no sign-in. The app works in a basement, a plant room or anywhere else with no mobile signal.
Free to use, one purchase to go further
The calculators are free to use. Flowtemp Pro is a one-time purchase, not a subscription. It removes ads, unlocks whole-house heat loss across every room, unlimited projects and your own logo on PDF reports.
Standards-based estimates
Heat loss follows BS EN 12831-1. Radiator output uses the EN 442 ΔT correction. Results are design estimates to help you plan. They do not replace manufacturer data, a full design or the regulations that apply to your work.