Free calculators for the trades
48 calculators built for UK electricians, gas engineers, heating engineers and plumbers. BS 7671-referenced where it matters, free to use, no sign-up — and all of them work offline in the TradePlanr app.
Popular calculators
Max Zs Lookup
Maximum earth fault loop impedance for BS EN 60898/61009 devices.
Voltage Drop
Volt drop for twin & earth over a run, checked against BS 7671 limits.
EV Charger Load Check
Whether an EV charger fits the existing supply, or needs load management.
Self-Employed Tax & NI
Estimate Income Tax, Class 4 NI and take-home from your annual profit.
Boiler / Heat Pump Sizing
Estimate whole-house heat loss and the boiler or heat pump size to match.
Gas Rate Calculator
Timed meter test — natural gas or LPG, metric or imperial — with live kW results.
Heating & Plumbing
Radiator / Room Sizing
Approximate heat requirement for a room from its size and use.
Expansion Vessel Sizing
Minimum vessel size for a sealed heating system.
System Volume Estimate
Approximate water content of a heating system.
Cylinder Reheat Time
Time and energy to heat a hot water cylinder.
Flow Rate from kW & ΔT
Required circulation flow for a heat load at a design ΔT.
Radiator ΔT Output Correction
Convert a radiator’s catalogue output to the output at your system’s ΔT.
Underfloor Heating Pipe Length
Estimate UFH pipe length and loop count from floor area and pipe spacing.
Boiler / Heat Pump Sizing
Estimate whole-house heat loss and the boiler or heat pump size to match.
Heat Pump vs Gas Running Cost
Compare the annual running cost of a heat pump against a gas boiler.
Electrical
Voltage Drop
Volt drop for twin & earth over a run, checked against BS 7671 limits.
Electric Shower Cable & MCB
Design current, MCB rating and typical cable size for an electric shower circuit.
Max Zs Lookup
Maximum earth fault loop impedance for BS EN 60898/61009 devices.
Zs from Ze + R1+R2
Calculated Zs for a circuit from external loop and conductor resistances.
R1+R2 from Cable Size
Expected conductor resistance for a run from OSG Table I1 values.
Adiabatic CPC Check
Minimum CPC size: S = √(I²t) / k.
Cable Sizing (Correction Factors)
Required tabulated capacity It ≥ In / (Ca·Cg·Ci·Cf), with a T&E suggestion.
Prospective Fault Current
Fault current from a measured loop impedance.
Ring Final Expected Readings
Cross-connected test values from end-to-end ring measurements.
Max Demand (Diversity)
Household maximum demand with OSG diversity allowances.
Ohm's Law / Power
Enter any two of V, I, R, P — the rest are calculated.
kW ↔ Amps
Convert between power and current for single or three phase.
Trunking Fill
Minimum trunking size for a bundle of singles (OSG factors).
EV Charger Load Check
Whether an EV charger fits the existing supply, or needs load management.
EV Charging Cost
What it costs to charge an EV at home — per charge, per mile and per year.
Solar PV Payback
Estimate annual savings and the payback period on a solar PV system.
Air Conditioning & Refrigeration
Air Conditioning Size
Estimate the cooling capacity a room needs.
F-Gas CO₂e & Leak Check
CO₂-equivalent tonnage and leak-check interval from the charge.
Air Con Running Cost
What an air conditioner costs to run using its SEER.
Superheat & Subcooling
Superheat and subcooling from your gauge saturation and line temperatures.
Refrigerant Pipe Sizing
Indicative liquid and suction line sizes for a split system by capacity.
General
Running Cost
What an appliance costs to run at a given tariff.
Unit Converter
Common trade unit conversions.
Markup & VAT
Sell price from cost, markup and VAT.
Job Price / Quote Builder
Build a quote from materials, labour and margin.
Day Rate ↔ Hourly Rate
Convert between hourly, day, week and year.
CIS Tax Deduction
CIS deducted and net pay on a subcontractor invoice.
VAT Flat Rate vs Standard
Compare VAT owed on the flat rate and standard schemes.
Self-Employed Tax & NI
Estimate Income Tax, Class 4 NI and take-home from your annual profit.
VAT Registration Threshold
How close your rolling turnover is to the £90,000 VAT registration threshold.
What Should I Charge?
Work back from the income you want to the day and hourly rate you need to charge.
Late Payment Interest
Statutory interest and compensation you can claim on an overdue commercial invoice.
Deposit & Staged Payments
Split a job into a deposit and staged payments.
Retention (Commercial)
Retention held on a contract and when each half is released.
Mileage / Fuel per Job
Mileage claim or actual fuel cost for a journey.
Real Hourly Rate
Your true hourly rate once travel, quoting and admin time are counted.
Get all 48 calculators in your pocket
The TradePlanr app puts this calculator — plus gas & electrical certificates, quoting, invoicing and job sign-offs — on your phone, working even with no signal. Free to start — works in your browser, no download needed.
