Consumer Unit Calculator — estimate consumer unit for your project. Fast, accurate, free. No signup required.
Consumer-unit planning starts by counting circuits, reserving spare ways, and checking the main-switch and incoming-supply capacity. The physical way count is only one part of selecting a safe and serviceable distribution board. The Consumer Unit calculator keeps the topic-specific quantities visible so you can audit the assumptions before using the result.
Construction estimates are highly sensitive to site conditions, product specifications, local policy, and the stage of design. A useful early estimate states its units, boundary, design allowance, and source of any reference value. It should help you ask a better question of a supplier, designer, building-control officer, planning authority, or installer—not pretend that a three-field calculation is a signed design.
Use the related construction calculator for an adjacent check and the companion standards calculator when the result crosses into quantities, structure, services, energy, water, or regulation. Keep the original dimensions and assumptions with the displayed output so a later revision can be compared on like-for-like terms.
Where the page gives a consent, fire, electrical, accessibility, heritage, or safety prompt instead of a single number, that is deliberate. Those topics depend on documents, inspections, local rules, tested products, and professional judgement. The calculator can organise the first pass, but it cannot issue an approval or replace a competent person's review.
Required ways = number of circuits × (1 + spare-way allowance ÷ 100), rounded up to a whole way.
The formula is intentionally shown beside the inputs. Convert millimetres, metres, kilowatts, amps, percentages, and category scores consistently, and keep intermediate precision until the final display. A plausible-looking answer can still be unusable if a clear opening was confused with an outside frame, a nominal capacity with a tested rating, or a planning assumption with a statutory threshold.
For site work, the formula is the beginning of the check rather than the complete specification. Add the conditions that control the real decision: subgrade and drainage for roads, grouping and installation method for cables, weather and emitter temperature for heat pumps, significance for heritage, occupancy for escape, and local policy for planning matters.
Inputs used here: number of circuits, spare-way allowance, and main-switch rating. Keep each input traceable to a drawing, survey, test result, product sheet, policy document, or clearly marked planning assumption.
The result gives an indicative board size and spare capacity. The final board must also accommodate RCBOs or RCD groups, surge protection, busbar arrangements, enclosure requirements, cable entry, heat dissipation, and the characteristics of the supply.
Check direction and order of magnitude before relying on the result. A larger opening should increase area, a longer cable should increase voltage drop, a higher COP should reduce electrical input, a wider exit should increase the indicative capacity, and a higher traffic class should demand a stronger cover or pavement solution. If a change behaves the other way, recheck the units and the field mapping.
Do not report more precision than the survey or input supports. Round a procurement quantity up where the supplier sells discrete sizes, preserve the unrounded design calculation for review, and label any result based on a default rate or simplified weighting. If the number affects structural stability, fire safety, electric shock protection, accessibility, legal consent, or a regulated rating, stop at the estimate and obtain the appropriate review.
Consumer-unit replacement and circuit additions require competent design, testing, and certification under the applicable electrical rules. This calculator provides preliminary educational guidance only and does not constitute a structural design, electrical certificate, fire strategy, planning decision, Building Regulations approval, heritage consent, accessibility audit, or certified sustainability assessment.