Home, DIY and garden calculators

What these tools are for

Nearly every job here starts with the same question: how much does this take. The Paint Calculator works from wall area and the number of coats. The Gravel & Aggregate Calculator turns a path or a drive into tonnes. The Decking, Fence, Roofing and Insulation calculators do the same job for their own trade, each in its own units.

Others answer the cost question instead. The Firewood & Heating Cost Calculator compares what a winter of heat costs by fuel. The Water Usage & Bill Estimator does the same for taps and showers. The Skip Size & Waste Volume Calculator helps you pick the skip before the rubble is already piled up on the drive.

A third group belongs in the workshop. The Board Foot Calculator measures timber the way a merchant sells it. The Cut List Optimiser packs your parts into stock lengths. The Stair Calculator works out rise and going. The Cable Size & Voltage Drop Calculator sizes a run out to the shed.

Why a browser suits this kind of work

This is the one category where you are rarely sitting at a desk. You are up in the loft, out in the garden, or standing in the aisle at a builders' merchant with a tape measure in one hand. Signal in those places is often poor. A page here loads once and then keeps working with the network switched off, because the sums happen on your own device.

The honest limit is that a calculator only knows the numbers you type in. It cannot see that the room is out of square, that the joists sit at odd centres, or that the ground falls away at one end. Treat the answer as a solid first figure, not a survey.

Rules also vary by country. Stair dimensions, cable sizes and insulation values are all governed by local building regulations. Use these tools to plan a job and to sanity-check a quote. For notifiable work, check the current rules where you live.

Before you start measuring

The commonest mistake is treating a space as one clean rectangle. Measure each section on its own and add the results. Deduct large openings, such as a patio door, but leave the small ones in the total. That extra usually covers your offcuts.

Then think in what the supplier actually sells. Paint comes in fixed tin sizes, gravel in bulk bags, timber in stock lengths. A result of 7.4 tins means eight tins. Round up once, at the end, rather than at every step along the way.

It helps to run the sum twice: once with your measurements, once with a little added. If both answers land in the same bag, tin or length, you can order with confidence. If they fall either side of a boundary, go and measure again first.

Rolls, drops and the last row

The tools that cover a floor or a wall do not answer in square metres. That is not laziness. None of these things is sold that way. Carpet comes off a roll of fixed width in one long piece, so what you buy is a length of roll, and that length depends on which way the drops run. Take a 5 by 6 metre room and a 4 metre roll. Run the drops the long way and you need two of 6 m, so 12 m of roll. Turn them and it is two of 5 m, or 10 m. The Carpet Calculator works it out both ways and shows you the one you did not pick. It will not simply choose the cheaper for you. Pile has a direction, and every drop in a room has to run the same way, or the two halves look like different colours under the same light.

Wallpaper turns on the pattern repeat, and that is the figure people leave out. At a 2.4 m ceiling a plain paper gives four drops from a 10 m roll, with 400 mm to spare. Now put a 640 mm repeat on it. Every drop has to be cut to a whole number of repeats, so 2.56 m rather than 2.4, and the same roll gives three drops. That is a third more rolls for the same wall. An offset match loses half a repeat again on alternate drops, and the Wallpaper Calculator allows for that on average. It also counts each door or window as one whole drop saved. That is how decorators work: the strip above a door rarely yields a full drop you can use.

Planks and tins each have a catch of their own. In the Laminate Flooring Calculator the number to watch is the width of the last row. Rip it under about 50 mm and it has nothing to clip to, so it lifts. The fix is to narrow the first row as well and split the shortfall between the two. The tool prints both widths, so you can decide before half the floor is down. The Paint Calculator answers in tins rather than litres. It also counts a mist coat on new plaster as its own line, at half the coverage printed on the tin, because a porous wall drinks about twice what a sealed one does. Painting over a much darker colour adds a coat instead.

The wastage figures are set to sensible starts and then left open to you. Laminate begins at 8 per cent, which suits a square room laid straight; 12 is nearer the mark for a diagonal lay or a room with bays. Carpet begins at 5 per cent, roof shingles at 10, bricks at 5, and mortar at 15. Every one of them is a field you can change. A laminate floor is also laid to a 10 mm expansion gap all round by default, and that gap comes off the room before a single plank is counted.

Nominal sizes, and the post at the end of the run

Many of the sizes a merchant quotes are names, not measurements. A two by four has been neither two nor four inches since it left the sawmill. It finishes at 1.5 by 3.5. Hardwood sold as 4/4, or four quarter, is a nominal inch that turns up at about thirteen sixteenths, and it is still billed as a full inch, because the mill sawed an inch and sold you the sawdust with it. The Board Foot Calculator carries both habits, since a board foot is a volume and not an area: one foot square by one inch thick, or 144 cubic inches. Pick the wrong habit and the bill is out by roughly 30 per cent.

Brickwork has the same gap between the name and the thing, and here it is worth understanding rather than looking up. A UK brick is 215 long, 102.5 wide and 65 high. Add the 10 mm joint and the module comes to 225 by 112.5 by 75. Those numbers are chosen so that two widths plus a joint equal one length: 102.5 + 10 + 102.5 = 215. That one identity is what lets headers and stretchers meet in the same wall, and the Brick & Block Wall Calculator tells you when a unit is not made to it and so cannot be laid in English or Flemish bond. The gauge falls out of the same module. At 75 mm a course, a 2.4 m wall is exactly 32 courses. Be a millimetre out on each joint and you are 32 mm out at the top, which is enough to miss a window head. American units go about it the other way: they are quoted nominally and made three eighths of an inch under, so that the unit plus its joint comes back to the round figure.

Two more numbers are not what they look like. A roof measured on the ground is not a roof. A pitch of 6 in 12 makes every metre of footprint 1.118 metres of surface, from the square root of 6 squared plus 12 squared, over 12, and that slope factor is the whole job of the Roofing Calculator. Aggregate is the other one. It is laid by volume and sold by weight, and the figures in the Gravel & Aggregate Calculator are bulk densities, measured loose with the air gaps in, which is how it arrives on the lorry. Granite is 2.7 tonnes per cubic metre as solid rock but about 1.5 loose as chippings. Use the solid figure and you nearly double the order.

The other quiet error sits at the ends of a run. Six fence panels need seven posts, because both ends want one. A stud wall needs the length over the spacing, rounded down, plus one, for the same reason. A deck board takes up its width plus its gap, so a 144 mm board with a 5 mm gap is really a 149 mm pitch, and across a 4 m deck that is a whole extra board. If the deck is longer than the boards on sale, every run needs a butt join, and a butt join has to land on a joist, so the joist count is not the span over the spacing either. Openings work the same way in reverse. A door in a stud wall adds timber rather than taking it away: a king stud and a jack stud each side, plus short cripple studs above, which is four extra pieces before the lintel itself.

One practical note. Every page in this category keeps the last numbers you typed in your own browser, each under a key of its own, so a page comes back the way you left it, and a reset button clears that store. The twenty-eight calculators go one step further and hand you the result as a plain text file or as an image, which is the version worth having on your phone at the trade counter. The Weekly Meal Planner is the one page here that is not a calculator, and it gives you the merged shopping list to copy or print instead.

Fourth powers, saw kerf and creep

A few of these tools do real engineering rather than multiplication, and each one turns on something intuition gets wrong. The Shelf Sag Calculator uses the standard beam formulas: 5wL⁴/384EI for a shelf on two supports, wL⁴/384EI for one held rigid at both ends, and wL⁴/8EI for a cantilever. The fourth power on the span is the brutal part. Move the brackets from 800 mm apart to 1000 and the sag more than doubles. Thickness is the kind one, because the stiffness of a rectangle goes with the cube of its depth. Going from 18 mm to 25 is only 39 per cent more timber, and it cuts the bend by nearly two thirds. A glued-on front lip does the same job for less, by moving wood away from the middle of the section, where it was doing nothing.

The same tool carries the fact nobody warns you about. Boards made of wood keep moving under a load that stays on them. Chipboard and MDF are given the Eurocode 5 creep factor of 2.25 for a heated indoor room. So the sag two years on is 3.25 times the sag on the day the books went up, and the shelf is checked against both. Metal and glass do not creep at room temperature, so theirs is zero. The stiffness figures behind all this are in newtons per square millimetre: chipboard 2,750, oak 12,500, glass 70,000, steel 200,000. Toughened glass is entered at the same figure as plain float glass. Toughening makes glass stronger, not stiffer, so a toughened shelf bends just as far as a plain one before it breaks.

Loads are taken per metre of shelf: 18 kg of paperbacks, 30 of hardbacks, 55 of vinyl records standing up. A metre of LPs weighs nearly twice what a metre of hardbacks does. A shelf on two supports is judged against its span over 360. A cantilever is judged against its projection over 180, which is the same test applied to twice the projection, since a shelf sticking out 300 mm reads like a 600 mm span. The Stair Calculator is strict in the same spirit. The riser count is a whole number and the height is the total rise divided exactly by it, never rounded step by step, because a flight with one odd step is where people fall. It then checks 2R + G, which wants to land between 550 and 700 mm.

The Cut List Optimiser has a saw blade to deal with. Kerf turns 3 mm of board into sawdust at every cut, which is why three 800 mm parts do not come out of a 2400 mm board. Packing parts into stock lengths is one-dimensional bin packing, and that problem is NP-hard. So the tool runs two rules of thumb, first-fit decreasing and best-fit decreasing, and keeps whichever answer used fewer boards. It also works out the floor: the total length of your parts over the board length left after trimming. It calls a plan optimal only when the plan matches that floor. Lines it could not read are counted and shown back to you rather than dropped, because a cut list that quietly loses a line is found out at the saw, with the wrong number of boards already bought.

It is worth knowing where these stop. The Cut List Optimiser packs lengths along a board; it does not nest parts on a sheet. The Insulation Calculator gives steady-state fabric loss through an area and nothing more: no ventilation, no cold bridges at junctions, no solar gain. It adds up the resistance of each layer and inverts once at the end, because R-values add across a build-up and U-values never do. It also counts the film of still air on each face, worth about 0.17 m²K/W for the pair, which is most of the resistance of a single sheet of glass. Leave those out and a thin build-up looks far worse than it is. The conductivities are in watts per metre kelvin, and PIR board at 0.022 against mineral wool at 0.038 means the same thickness gives nearly twice the resistance.