UK Home Improvement & Building Calculators

DIY 123 is a set of free calculators and plain-English guides for home improvement in the UK. Most of the tools answer the question you face before a trip to the builders' merchant: how much concrete, how many bricks, sheets of plasterboard, tiles or litres of paint. Others help with bigger decisions, such as whether a rear extension or loft dormer is likely to fall within permitted development in England, what size radiator a room needs, or how a heat pump's running costs compare with a gas boiler's. Each calculator shows its formula and a worked example, and says where its estimate stops being reliable.

Formulas shown on every page Metric UK trade sizes Planning rules for England

Start here

Select your project scenario below for immediate material estimates, dimensional thresholds, and trade guidance.

Planning an extension or loft conversion

Check permitted development limits, joist quantities and concrete volumes before starting work on extensions or dormers.

Decorating and interior renovation

Estimate trade quantities for stud partitions, plasterboard sheets, wall tiles, and paint coverage to reduce waste and trips to merchants.

Cutting heating bills and energy efficiency

Assess heat pump running costs against gas boilers, calculate radiator heat outputs, and evaluate loft insulation depth savings.

Electrical and EV charging

Calculate circuit cable voltage drop under BS 7671 standards and check dedicated supply requirements for domestic electric vehicle chargers.

Looking for our full directory across all building trades? See all calculators

Latest guides

View all 20 guides →

Practical explanations of UK planning thresholds, joist notching limits, heat loss formulas, and permitted development rules.

Tiling Bathroom Walls: Preparation, Quantities and Waterproofing
How to prepare, set out and tile bathroom walls in a UK home: substrates, tanking, adhesive and grout quantities, ventilation and electrical points.
Read guide
Which Home Projects Need Building Regulations Approval?
A plain guide to what building regulations cover in England, which common projects need approval, how competent person schemes work and what happens if you skip it.
Read guide
C16 vs C24 Timber Joists and the Rules on Notching and Drilling
What the C16 and C24 strength classes mean, how to choose joist sizes, and the accepted limits for notches and holes for pipes and cables.
Read guide
Damp, Mould and Condensation in UK Homes: Diagnosing the Real Cause
How to tell condensation from penetrating and rising damp, what a moisture meter can and cannot tell you, ventilation fixes, and when to involve a landlord or surveyor.
Read guide
Installing a Home EV Charger in the UK: Circuit, Smart Rules and Grants
What a home electric vehicle charger installation involves in the UK: dedicated circuit, earthing, smart charging regulations, planning, grants and who can fit it.
Read guide
Building a Garden Brick Wall: Height Limits, Foundations and Materials
Plan a freestanding brick garden wall in the UK: permitted height, foundations, bricks, mortar, piers, copings and how to count bricks and mortar.
Read guide

How we work out the numbers

Most calculators use standard UK trade sizes and simple geometry; where a figure comes from an Approved Document or a British Standard, the page names it. Dimensional estimators apply standard UK brick dimensions (215 × 102.5 × 65 mm) and nominal plasterboard sheet coverage. Electrical calculations follow the millivolt-per-ampere-per-metre voltage drop tables in BS 7671, while heating estimates align with standard room temperature baselines. We do not accept sponsored material rates or commercial endorsements. To read our complete technical methodology and source citations, review our editorial policy or discover more about our project.