Electrical Tools & Calculators
Free BS 7671 compliant calculators for UK electricians. Accurate, fast, and built for real-world electrical installations.
12
BS 7671 Tools
8
Circuit Design
24
Specialized
Free
All Tools
Newest calculators
BS 7671 compliance tools for testing and design
Earthing & Bonding Calculator
Max Zs Calculator
Ring Final Circuit Calculator
Diversity & Maximum Demand Calculator
Prospective Fault Current Calculator
Three-Phase Load Balancing Calculator
Lighting Circuit Design Calculator
Motor Circuit Calculator
Google Visibility Checker
UPS Sizing Calculator
Generator Sizing Calculator
BS 7671 Calculators
Compliance tools for testing, design verification, and regulatory calculations
Earthing & Bonding Calculator
Calculate main earthing, protective bonding, and CPC sizes using BS 7671 regulations
Max Zs Calculator
Calculate maximum earth fault loop impedance for MCBs and fuses per BS 7671 Table 41.3
Ring Final Circuit Calculator
Calculate continuity test values and verify ring circuit design according to BS 7671 Appendix 15
Diversity & Maximum Demand Calculator
Calculate maximum demand with diversity factors for all premises types per BS 7671 Regulation 311.1
Prospective Fault Current Calculator
Calculate PSCC and PEFC to determine breaking capacity requirements per BS 7671 Regulation 612.11
Installation Cost Estimator
Estimate costs for electrical installation work across the UK with 2025 pricing
Three-Phase Load Balancing Calculator
Balance loads across L1, L2, L3 phases with phasor-based neutral current calculation and auto-balancing
Lighting Circuit Design Calculator
Calculate lighting circuit capacity, cable sizing, voltage drop, and diversity for LED, CFL, fluorescent, and halogen circuits
Motor Circuit Calculator
Calculate FLC, size cables, and select protection devices for DOL, Star-Delta, and Soft Start motor installations per BS 7671 Regulation 552
Earth Fault Loop Impedance Calculator
Calculate Zs from Ze + R1+R2 and verify against BS 7671 Table 41.3 maximums with the 80% rule
Maximum Demand Calculator
Calculate whole-building maximum demand, ADMD for developments and main fuse sizing per BS 7671
R1+R2 Calculator
Calculate R1+R2 with temperature correction and Zs compliance per BS 7671 Table I1
Circuit Design Calculators
Essential tools for designing and sizing electrical circuits
Cable Size Calculator
Calculate the correct cable size for UK electrical installations with BS 7671 compliance
Voltage Drop Calculator
Calculate voltage drop for single and three-phase circuits with BS 7671 compliance checks
Load Calculator
Calculate electrical loads and circuit breaker sizing with diversity factors
Circuit Breaker Selector
Select the correct MCB or RCBO for your circuit
Cooker Circuit Calculator
Size cable, MCB and voltage drop for cooker circuits using the IET diversity formula
Shower Circuit Calculator
Size cable, MCB and Zs for 7.5–10.8kW electric showers per BS 7671
SWA Cable Size Calculator
Size SWA cable with gland selection, CPC verification and derating factors per BS 7671
3-Phase Cable Size Calculator
Size three-phase cable for SWA, singles and conduit with voltage drop and neutral sizing
Converters & Fundamentals
Quick electrical conversions — watts, amps, kW, kVA, AWG and Ohm's Law
Watts to Amps Calculator
Convert between watts and amps for single and three-phase circuits
Amps to kW Calculator
Convert current to real power (kW) and apparent power (kVA)
kW to Amps Calculator
Convert kilowatts to current for sizing breakers and cables
kVA to kW Calculator
Convert between apparent power (kVA) and real power (kW)
AWG to mm² Calculator
Convert American Wire Gauge to metric cable sizes and back
Ohm's Law Calculator
Solve for voltage, current, resistance or power from any two values
Specialized Calculators
Purpose-built tools for specific electrical applications
Google Visibility Checker
Free website audit for electricians — see how well your business shows up on Google and what to fix to win more local work
Immersion Heater Calculator
Size cable, MCB and isolator for a dedicated immersion heater circuit
Heat Pump Electrical Calculator
Size supply cable, MCB type and isolation for an air-source heat pump
Hot Tub Cable Calculator
Size SWA/T&E cable, MCB and 30mA RCD for an outdoor hot tub
Fire Alarm Cable Calculator
Check loop voltage drop and size fire-resistant cable per BS 5839-1
Power Factor Correction Calculator
Size the capacitor bank (kVAr) to improve power factor and cut current
SWA Gland Size Calculator
Find the correct gland size, thread and torque for SWA cable
RCD Selector
Choose the correct RCD type and sensitivity for a circuit per BS 7671
DNO Application Checker
G98 or G99? Check what your solar, battery or EV install needs
Discrimination Checker
Check MCB/fuse discrimination and selectivity per BS 7671 536.4
Transformer & Busbar Calculator
Size a distribution transformer, fault level and copper busbar
Emergency Lighting Calculator
Luminaire count, spacing and battery capacity per BS 5266-1
Consumer Unit Configurator
Size the board, main switch and protection, with a circuit schedule
Smart Home Load Calculator
Total device load, PoE budget, UPS sizing and running cost
Test Results Checker
Check measured Zs, insulation resistance and RCD trip times per BS 7671
UPS Sizing Calculator
Calculate UPS capacity, battery requirements, and costs for critical power protection
Generator Sizing Calculator
Size backup generators with fuel consumption and cost estimates for all applications
EV Charger Calculator
Calculate electrical requirements for electric vehicle charging installation
EV Charging Cost Calculator
Work out the cost to charge an EV at home or public chargers, with cost per mile and a petrol comparison
Solar Panel Calculator
Size photovoltaic systems and calculate generation, costs, and payback
LED Savings Calculator
Calculate energy and cost savings from switching to LED lighting
Conduit Fill Calculator
Determine how many cables can fit in conduit according to BS 7671
Cable Tray & Trunking Calculator
Size cable tray and trunking with BS 7671 space factor compliance, segregation warnings, and support spacing per BS EN 61537
Wire Length Calculator
Calculate total cable length and quantities for electrical projects
Reference Guides
Interactive guides and reference materials
Why UK Electricians Trust Our Calculators
Built to meet the real demands of site work — not to tick boxes, but to help you do the job right.
We Show the Working
No "black box" calculations. Every result includes the methodology—see the correction factors (Ca, Cg, Ci) applied, the tables referenced, and the formula used.
If a cable recommendation "looks wrong", you can see exactly why and sanity-check the inputs.
Explicit BS 7671 References
Results reference specific regulation numbers and tables—Table 4D5 for T&E, Table 54.7 for earthing conductors, Regulation 612.11 for PFC. Cross-reference with your own copy.
Updated for BS 7671:2018+A2:2022 (Amendment 2) requirements.
Clear Pass/Fail Indicators
Traffic light compliance status at a glance. Green for compliant, amber for borderline (approaching limits), red for non-compliant—with specific reasons shown.
"Voltage drop exceeds 3% (11.5V)" is actionable. "Error" is not.
Built for Site Use
Large touch targets for gloved hands. High-contrast interfaces readable in bright sunlight or dark plant rooms. Dropdowns and toggles instead of fiddly typing.
No login walls. Works instantly in any browser.
Real Installation Methods
We don't ask vague questions like "Is it in a wall?" We ask for the specific reference method—Method 100 vs 101 vs 102/103. Because a cable in thermal insulation isn't the same as clipped to a joist.
Current capacity can drop 50%+ depending on installation—we model this correctly.
Free. No Subscriptions.
Ohm's Law hasn't changed in 100 years. You shouldn't pay monthly for basic physics. All our calculators are free—no accounts, no paywalls, no "upgrade for more features".
Bookmark and use. That's it.
Designed for Your Workflow
Our calculators are built to support—not replace—your professional judgment. Use them for sanity-checking designs, quick on-site verification, or training apprentices on the "why" behind cable sizing. The methodology is always visible so you can explain your design decisions to clients, inspectors, or trainees. Results can be printed or saved as part of your project documentation.
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