INDEX / GUIDES
Guides
Longer explanations that do not fit inside a calculator: where the default assumptions come from, what changes them, and the mistakes that make people buy the wrong quantity.
How much paint a bedroom actually needs
A worked example from tape measure to till, using a real 4 × 3.5 m room, with every number shown and every assumption named. If you have ever come home with a tin and a half too many, this is where the extra half came from.
Paint coverage by surface, and why porous walls drink more
The number on the tin is one number for every wall in the world, which is obviously impossible. This is what actually changes coverage, by how much, and how to pick a realistic figure before you buy rather than after you run out.
When primer is worth it, and when it is not
Primer is either the cheapest insurance in decorating or a wasted afternoon, and which one it is depends entirely on what you are painting onto. Here is how to tell the two cases apart before you buy a tin you do not need.
Tile waste percentage by laying pattern
Everyone says "add ten percent". Ten percent of what, and why ten? Waste is not a property of the tile — it is a property of your room, your pattern and the person cutting. Here is how to pick a number you can defend.
Measuring a room for wallpaper when the pattern repeats
Wallpaper is the one material where the number on the label — ten metres a roll — tells you almost nothing about how far it goes. The pattern repeat decides that, and it can cost you a third of every roll.
Metric and imperial for building materials, without conversion errors
Mixed units are where quiet, expensive mistakes live: a gallon that is not a gallon, a coverage figure converted the wrong way round, a tile sold in inches for a room measured in metres. Here is how to keep them apart.
How many coats of paint do you actually need?
The tin says two. Sometimes one is enough and you have wasted a day; more often three is the honest answer and you run out of paint on a Sunday afternoon. The number of coats is the single biggest multiplier in your estimate, so it is worth getting right before you buy.
How many tiles per square metre, by size
A shop quotes tiles by the box, a tiler quotes by the square metre, and the label gives you dimensions in centimetres. Converting between the three is where most ordering mistakes happen — and unlike waste, this part is exact arithmetic with a right answer.
How many bags of concrete per cubic metre
The answer surprises people: around eighty 25 kg bags to fill one cubic metre. That is two tonnes of material, more than a small car weighs, and it is why the bag-versus-truck decision is usually made for you by the volume rather than by preference.
Underlay, beading and skirting: the quantities people forget
You worked out the flooring and ordered it. Then the fitter asks about underlay, expansion gaps, threshold bars and how many lengths of beading — and suddenly the job has four more line items, each with its own way of being counted. Here is all of it in one place.
How deep should topsoil, compost and mulch be?
Garden materials are sold by volume but spread by depth, and depth is the input everyone guesses. Two centimetres too deep across a lawn is an extra half cubic metre — half a tonne you carried up the drive for nothing.
How to measure a room for materials
Every calculator on this site starts with the same four numbers: length, width, height and openings. Get them right and the arithmetic is trivial. Get them casually and every estimate downstream inherits the error — which is why this is the one guide worth reading before any of the others.