Free BS 7671 calculators for UK electricians.
Cable sizing, voltage drop, Zs, disconnection times, maximum circuit length, PFC, conduit fill. The calculations you do every day, built against the latest IET Wiring Regulations. No login. No subscription. Works on your phone on site.
TradeCalcs is a free PWA offering BS 7671-compliant electrical calculators for UK electricians — cable sizing, voltage drop, Zs, disconnection times and more. No login, ad-supported, built for site use.
The calculations UK electricians actually use.
Built from real on-site work, not a rewrite of a textbook. If it is something you calculate on a job, it is here.
Cable Sizing Calculator
Select the correct cross-sectional area for the load, installation method and ambient conditions. Handles both single-phase and three-phase.
Voltage Drop Calculator
Verify volt drop against BS 7671 Appendix 4 limits for the circuit run. Takes account of cable length, current, and conductor resistance.
Maximum Circuit Length
Work out the longest run that still meets Zs and volt drop requirements simultaneously. Prevents the "passes one, fails the other" trap.
Zs Earth Fault Loop
Compare measured Zs against the BS 7671 maximum for the protective device, with automatic temperature correction for 80% rule compliance.
Disconnection Time
Check automatic disconnection is achieved in the required time for the circuit type — 0.4s or 5s depending on the circuit and installation method.
Prospective Fault Current
Calculate PFC from the supply characteristics for circuit design and breaker selection. Essential for specifying breaking capacity.
Conduit Fill
Work out conduit and trunking capacity for the cables being installed. Follows BS 7671 recommended fill ratios.
Unit Conversions
Amps, watts, volts, kVA, kW, power factor — the everyday conversions electricians actually need, without the ads of generic calculator sites.
Why TradeCalcs is built differently.
Most electrical calculators online are either locked behind a subscription, built for US wiring, or pulled from a textbook with no thought for how you actually use them. TradeCalcs is the opposite.
Built for the van, not the office.
Every calculator is mobile-first. Large tap targets, no tiny desktop-only forms, no hunting for the submit button with cold fingers. Works in a roof space, up a ladder, kneeling on a fuse board — wherever you actually do the work.
UK regulations, not US wiring.
TradeCalcs follows BS 7671 exclusively. No NEC equivalents, no imperial units, no calculators that assume 120V residential circuits. If the input boxes and the formulas match what you were tested on for the 18th Edition, it is the right tool.
No subscription. No login. No upsell.
You do not need to create an account, provide an email, or agree to a trial. Open the site, pick the calculator, run the numbers. The ads are there to keep it free — we would rather have the tool used than fenced behind a paywall that nobody needs for basic calculations.
Updates tracked to the regulations.
When the IET publishes amendments, the calculators get updated. We review every tool against the new text and patch formulas, tables, and validation rules. You do not have to reinstall anything — the PWA updates itself.
Why we built TradeCalcs.
Most electrical calculators online are either locked behind a subscription, built for US wiring (which does not apply here), or pulled straight from a textbook with no thought for how you actually use them in a van or up a ladder.
TradeCalcs is the opposite. It is mobile-first, because that is where you are. It is BS 7671, because that is what governs your work. It is free, because every UK electrician should have the basics to hand without paying a subscription for sums they did in college.
If you want something more — voice-driven certification with a full audit trail — that is what CertVoice is for. TradeCalcs covers the arithmetic. CertVoice covers the certification.
Built by someone who knows the site.
Built by someone who knows what goes wrong on site.
Autaimate’s founder spent 40 years in UK construction and trades before building TradeCalcs. The calculator list reflects what working electricians actually reach for — not what a textbook author thought they ought to need.
Used daily across the UK.
TradeCalcs is not a concept demo or portfolio piece. It is actively used by working electricians across the UK every day. If a calculator was wrong, we would hear about it — which is why the ones that stay in the toolkit are the ones that stand up to site use.
Ad-supported, not data-harvesting.
The ads keep the service free. That is the business model — not selling user data, not serving electricians as a captive audience to some larger platform. No logins, no tracking, no newsletter signup required.
Common BS 7671 calculation mistakes TradeCalcs catches.
The pitfalls that trip up both trainees and experienced electricians. The calculators flag them automatically so you do not sign off something that will come back on you.
Forgetting the 80% rule on measured Zs
Published Zs values in BS 7671 are for conductors at 70 degrees C. On a cold install the measurement will be lower than the eventual warm value. The rule of thumb is the measured Zs should be no more than 80% of the published maximum to allow for heating. TradeCalcs applies this correction automatically when you enter a measured value.
Volt drop passes but Zs fails
A long cable run can satisfy BS 7671 Appendix 4 volt drop limits while still exceeding the maximum Zs for the protective device. Both checks must pass independently. TradeCalcs runs both and tells you which one is pulling the run length down so you know exactly where the constraint is.
Wrong disconnection time for the circuit type
BS 7671 requires 0.4s disconnection on final circuits up to 63A in TN systems, and 5s on distribution circuits. Using the wrong target silently causes installations to pass calculations they should fail. TradeCalcs asks what the circuit is and applies the correct target.
Current capacity corrected for ambient and grouping
A 32A cable rating on a tag does not mean 32A in every installation. Ambient temperature above 30 degrees, bunching with other circuits, and thermal insulation all reduce the true capacity. TradeCalcs prompts for these factors and applies the correction — so the sized cable actually carries what you think it does.
Frequently asked questions
Are the TradeCalcs calculators BS 7671 compliant? +
Yes. Every calculator is built against the latest BS 7671 (IET Wiring Regulations, 18th Edition including Amendment 2) — cable sizing, voltage drop, maximum circuit length, Zs values, disconnection times and more all follow the regulations as written. We update the tools when amendments are published so calculations stay current with the standard.
Is TradeCalcs free to use? +
Yes, the calculators are free. The platform is supported by ads, which keeps it available to every electrician in the UK without a subscription or login. No trials, no upgrade prompts — the tools work the same whether you use them once a year or ten times a day.
Which calculations are included? +
Cable sizing, voltage drop, maximum circuit length, earth fault loop impedance (Zs), disconnection time verification, prospective fault current, conduit and trunking capacity, and the everyday unit conversions used on UK electrical installations. New calculators are added as electricians request them.
Does TradeCalcs work on phones and tablets? +
Yes. TradeCalcs is a Progressive Web App (PWA), so it works on any modern browser on phones, tablets, laptops and desktops. You can add it to your home screen so it behaves like a native app without going through an app store. It also works on site when you have a signal — and caches the interface for fast loading.
Who is TradeCalcs for? +
UK electricians, electrical contractors, trainees working towards 18th Edition qualifications, and anyone signing off electrical installation work against BS 7671. Also widely used by assessors, trainers, and domestic installers who need a quick reference during inspections or training sessions.
Can I trust the results for certification purposes? +
The calculators apply the BS 7671 formulas correctly, but the responsibility for the design and certification of an installation always rests with the qualified electrician. TradeCalcs is a working tool — not a replacement for competence, site inspection, or the full design process. Treat it the way you would a good slide rule or calculator — it does the arithmetic, you do the engineering.
Does TradeCalcs store my calculation history? +
No, TradeCalcs is a stateless tool. It does not store your calculations, does not require login, and does not track individual electricians. This is deliberate — the calculators are a reference, not a record-keeping system. For documented electrical certification with a full audit trail, use CertVoice (voice-first EICR, EIC and Minor Works) which is built specifically for certification and compliance documentation.
Can I use TradeCalcs without an internet connection? +
The initial load requires connectivity so the PWA can install. After that, the core calculators work on cached data — you can run calculations without signal. Anything that relies on live data updates (for example, if we publish amendments to cable tables) will sync the next time you are online.
How does TradeCalcs compare to commercial electrical design software? +
Full commercial design packages cost hundreds or thousands of pounds and are built for specification engineers doing complex system designs. TradeCalcs is built for working electricians who need fast, accurate calculations for everyday installation work. Different job, different tool, different price point — free rather than licensed.
What happens when BS 7671 is updated? +
Calculators are updated to match amendments as they are published. When IET releases a significant amendment (like Amendment 2 in 2022), we review every calculator against the new text and update the formulas, tables, and validation rules. The changes go live without you needing to update anything — it is a PWA, not an app store download.
Is there a way to report a bug or suggest a new calculator? +
Yes. Feedback goes to the Autaimate team and gets reviewed. The most-requested calculators get prioritised for the next release. Most of the current tool set came from electrician requests — if you need something the site does not cover, tell us and it usually ends up built.
Does TradeCalcs cover three-phase installations? +
Yes. Cable sizing, voltage drop, PFC and disconnection time calculators all handle both single-phase and three-phase installations. The interface asks for the system type up front and applies the correct formulas automatically — so you do not have to remember which equation changes with phase count.
Where to go next
CertVoice — voice-first electrical certification
If you need BS 7671-compliant EICR, EIC and Minor Works certificates with a full audit trail, not just calculations.
InspectVoice — voice-first inspection
BS EN 1176 playground inspections captured by voice, offline, with an on-device PDF at the end.
Construction & Trades products
All Autaimate products built for UK construction, trades and site businesses.
Ready when you are.
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