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Thirty-six calculators, and every figure in them is named

The arithmetic in these is trivial — an area, a volume, a rate. **The figures are the whole tool.** How far a litre of paint goes on bare plaster, what a cubic metre of wet sand weighs, how much heat a radiator gives at a heat pump's flow temperature, how much of a lead battery you can actually use: those are the numbers somebody came for, and getting one of them wrong is a wasted afternoon and a second trip.

So every published figure here is a **typical** one and says so on the result. The bag, the tin, the plate in the door frame and the manual all win, and each of these says which of them to go and read. A trade reserve is shown as a convention with the quantity without it beside it, because ten per cent is a habit rather than a measurement.

**Nothing here knows where you are.** No weather, no prices, no local rules, no building regulations. Every number is one you typed or one printed on the page, and certainty is not on offer: a heat loss worked out from six fields is not a heating design, and a snow load is not a structural assessment.

**The tables are metric, and the refusals have numbers in them.** A wall whose openings are bigger than the wall, a fence whose posts are further apart than the fence is long, a flow temperature below the room it is heating — each is refused with the figure that made it impossible, rather than answered with something plausible.

Building

Tile calculator
Tiles for a wall or a floor, joints and cutting waste included.

Warmth and power

Outside

Moving in

The car

The journey

Jet lag planner
A day-by-day shift before a flight, with the light that makes it work.

Questions

Why does the paint tool ask what the wall is made of?
Because it is the single biggest number in the calculation and the reason people run out. Bare plaster drinks about a third more than a wall that has been painted before, and new plasterboard more again — so a room worked out at the coverage printed on the tin comes up short by most of a tin.
Why does the firewood tool ask about moisture rather than just volume?
Because wet wood gives a fraction of the heat, and it is the most useful thing the tool can tell you. Water in the wood is boiled off before anything else burns: freshly cut, about half the heat of seasoned wood goes into steam. Every per cent of water above fifteen costs roughly one and a half per cent of what you get out.
Why does the radiator tool talk about heat pumps?
Because a radiator's catalogue output is measured at a flow temperature of seventy-five degrees, and a heat pump runs at forty-five or lower. Output goes as the temperature difference to the power 1.3, so the same radiator gives about a third of its rating — which is the whole reason a conversion needs bigger ones.
Why will the towing checker not just say whether I am allowed to tow it?
Because that is a licence question and a country's rule, not a physical one, and nothing here knows which country you are in. What it does is arithmetic against the limits on your own plate: it checks each one you give it, and says plainly which ones it did not check rather than calling them passed.