Free Home & DIY Calculators

Work out how much to buy before you start the job

Most DIY jobs go wrong at the shop, not on the wall: too little paint for a second coat, one box of tiles short, or twice the concrete you needed. These free home calculators give you the quantity first. The paint calculator works out litres from room size, coats and the doors and windows you subtract; the wallpaper calculator allows for pattern repeat; the tile calculator and flooring calculator turn an area into boxes with a sensible wastage allowance; and the concrete calculator gives cubic metres or bags for slabs, footings, posts and steps.

Outside the house, the brick calculator counts bricks or blocks and mortar for a wall, the roofing calculator converts pitch and footprint into roof area, sheets or shingles, the fence calculator lists posts, rails, panels and concrete for a run of fence, the deck board calculator counts boards and screws for a deck, and the stair calculator turns a total rise into an even number of risers and treads that meet common building-code limits.

Each page runs entirely in your browser, with metric and imperial units, and shows its working so you can check the assumptions against the product label. Homeowners, renters refreshing a flat, trades quoting a small job and students on construction courses use them; the result is a shopping list, not a guess.

Comfort, light, water and power

• Cooling. The AC tonnage calculator sizes an air conditioner in tons or BTU from room area, ceiling height, sun exposure, occupants and appliances, so you avoid a unit that short-cycles or never catches up.
• Lighting. The lumens calculator turns room size and the task (kitchen worktop, bedroom, workshop) into the total lumens you need and how many bulbs or fittings that is.
• Rain. The rainwater harvesting calculator estimates how many litres a roof can collect per month from its area and local rainfall, and what tank size makes sense.
• Solar. The solar panel calculator estimates the system size, number of panels and roof area for your monthly electricity use and sun hours, with a rough payback period.
• Will it fit? The furniture fit checker compares a sofa, fridge or wardrobe with your doorway, stairwell and the room itself, including the diagonal, before you order it.

Measuring and buying tips

• Measure twice and enter the figures in the same units throughout; mixing feet and metres is the most common cause of a wrong quantity.
• Keep the wastage allowance. Ten percent extra tiles or flooring covers cuts, breakages and a matching batch for repairs later; diagonal layouts need 15 percent.
• Paint coverage on the tin assumes a smooth, sealed surface. New plaster, bare wood and textured walls drink more, so choose the lower coverage figure.
• For concrete, the bag count assumes the mix on the bag; ready-mix is sold by the cubic metre, so round up to the supplier’s minimum delivery.
• Stair rise and going limits differ by country. The stair calculator flags steps outside the usual range, but check your local building regulations before you cut timber.