New tool: a Paint Calculator that works out how many coats you actually need

General
1

First tool in a new Home & DIY section, working out how much paint a room actually needs.

Most paint calculators I looked at before building this one ask you to type in a number of coats and hand back litres. This one works the other way round, it starts from two standard coats and adds a coat for a big colour change, light to dark or dark to light, or for exterior work in a coastal or high humidity area, and shows you exactly why it recommended what it did. You can still override it if your painter says otherwise.

It also fixes something a few generic calculators get wrong for South Africa, paint here is sold in 1, 5 and 20 litre tins, not 10 litre. The calculator works out the cheapest combination of those three real sizes for your exact litre count rather than just rounding up to the nearest big number.

Coverage rate is not one generic figure either. A new, bare plaster wall soaks up a lot more paint than one that is already painted and in good condition, and needs a primer coat first, while a textured exterior coating covers far less per litre than a smooth interior sheen. Those rates come from published South African paint technical data sheets rather than a single marketing number.

You can add a ceiling, priced separately since it uses the room’s floor footprint rather than wall area, and any extra wall, alcove or standalone surface that does not fit a simple rectangular room. Prices default to a mid-range PVA but every field is editable to your own quote.

More Home & DIY tools are planned, this is just the first. Let me know if the coat logic or the tin sizing looks off for a job you have actually quoted.

2

Now this is a handy one. I’ve painted more rooms than I care to remember and the big mistake people make is buying one coat’s worth and then standing in the shop again on day two because the colour didn’t cover. Dark colour over light wall, or light over dark, you’re doing two coats minimum, sometimes three if it’s a bold red or navy, no calculator needed to know that but nice that yours works it out properly with the actual area.

One thing I’d add for anyone reading this before they buy, don’t skimp on the primer if the wall’s had damp or salt staining, which happens a lot here on the coast with the sea air getting into plaster. Paint over that without sealing it first and you’ll see the stain bleed through again in a few months, guaranteed. Also ceilings need their own calculation, people always forget the ceiling eats paint differently to walls. Good little tool though, will use it next time I do the garage.

3

Good tips, thanks MarkD. The primer point is a good one, I didn’t build damp/salt staining into the calc since that’s more of a prep-work call than a paint quantity one, but it’s worth flagging for anyone painting near the coast.

Ceilings I did split out separately in the tool, so that should already be covered. Will keep the two/three coat thing in mind if I add a “dark over light” warning down the line.

Let us know how the garage turns out.