Calculators that show their working
3 tools, and more as they earn their place.
Mulch and soil
How many bags of compost, bark or topsoil a bed needs, in cubic metres and litres as well.
Grass seed
How much seed a new lawn or an overseeding wants, at the rate your packet actually gives.
Paint
How many litres and how many tins a room needs, once the doors and windows are taken off.
Why these show their working
A calculator that gives you a number and nothing else is a calculator you have to trust. These give you the number and then the three or four lines that produced it, in the same units you typed, so you can see where a figure came from and argue with it. Every one of them is a sum you could do on the back of an envelope, and on each page there is a short section showing how to do exactly that, along with a table of the common cases.
The inputs that matter most are the ones nobody can guess for you: the depth you want a mulch, the rate printed on your packet of grass seed, the coverage printed on your tin of paint. Those are inputs here rather than assumptions buried in the code, which is why the answers change when you change them. The starting values are reasonable defaults and nothing more.
Everything works in metres by default and switches to feet with one button, and the choice is remembered on this device alongside your other settings. Materials sold by the litre or the gram stay in litres and grams whichever you choose, because that is how they are actually sold, with the imperial equivalent shown beside the answer.
Checking the answer
Every one of these rounds up at the last step, because two thirds of a bag of compost is not something you can buy and running out at the far end of a border on a Sunday afternoon is worse than having a bag left over. Nothing else is rounded until the end, so the working you see is the working the sum did rather than a tidied version of it.
What none of them do is quietly add a safety margin. If you want ten per cent extra for settling or for the wall that turns out to be more porous than it looked, add it yourself, and the page tells you where that is worth doing. A tool that pads its own answer is a tool you cannot check against the back of an envelope, and being checkable is the point of showing the working at all.