In this guide 6 sections
Ask for wall area, openings and coats. Read coverage per litre from the data sheet. Multiply, add a waste allowance and round up to the pack sizes you stock. Then save every input and the coverage rate on the order, so a short supply can be explained rather than argued about.
Start with the few questions a buyer can answer
Ask for room length, width and ceiling height, plus the number of doors and windows, and let a buyer who already knows the wall area type it in. Offer square metres and square feet. Keep the choice on the first screen.
Then ask about the surface. New plaster, a painted wall, bare wood and bare metal all drink paint at different rates, and the sum must know which it is dealing with. New plaster wants a thinned mist coat first. Ask for the old colour too, because covering a dark wall with a light shade takes extra coats and sometimes an undercoat. Use dropdowns. Free text breaks the sum.
Ceilings, walls, skirting boards and doors take different finishes, from flat matt emulsion to satin gloss, so let the buyer add each as its own line. One calculator, several lines, one basket.
Store coverage once, on the product record
Everything rests on the coverage figure, which data sheets often call the spread rate. Take it from the maker’s technical data sheet and keep it on the product itself, never inside the calculator code, so that a revised sheet means changing one field.
Store a low figure and a high one, because a smooth sealed wall takes less paint than rough plaster and the shop must not promise the lower number. Store the recommended coats. Note whether a primer or sealer is advised, and record the date of the data sheet you copied from.
Pack sizes live there too. Offer only the sizes you hold.
- Coverage per litre, low and high, by surface type
- Recommended number of finish coats
- Primer or sealer advised, and which product
- Pack sizes stocked, with a stock count per size
- Name and date of the data sheet the figures came from
Do the sum, then round to what you can pack
The sum runs in five steps. Take the wall area and subtract the openings. Multiply by the number of coats. Divide by the coverage per litre. Add a small allowance for waste at the tray and for cutting in at the edges.
Now round. Suppose the sum comes to eleven litres and you stock tins of one, four, ten and twenty litres, which leaves the shop choosing between a few combinations. Show the closest cover, a ten plus a one, and show a ten plus a four for the buyer who wants a margin.
Tinted paint adds one more rule. Tins for a single room should come from one mixing run, or be boxed and mixed together, so the wall shows no join between batches. Say so on the result screen.
Keep the assumptions with the order
Save the inputs, not only the answer. When a painter rings to say she is a litre short, the first question is what was entered, and without the inputs nobody can answer it.
Print the same facts on the order confirmation and the packing slip, and keep them on the order record that shop staff see. A buyer who typed twenty-eight square metres for a room that measures thirty-five can see the gap for herself. The argument becomes a reorder.
Let the buyer correct a figure and recalculate before she pays, with the new inputs replacing the old ones. The earlier version stays in the history.
- Room or wall area, openings and the unit entered
- Surface, colour change and number of coats used
- Coverage figure applied and the data sheet date
- Litres before rounding and the pack combination chosen
- Whether the buyer edited a figure, and when
Test it on finished jobs before you publish
Take five jobs your painters have finished. Enter their room sizes, surfaces and coats, then compare the estimate with the litres they actually used, and note the gap for each room. Include one awkward room, such as a stairwell or a bedroom lined with built-in cupboards that cut away wall.
If the gap keeps pointing the same way, adjust the waste allowance or the coverage range, because those two fields carry every assumption the sum cannot see. Leave the sum alone. Word the result as an estimate, put a phone number beside it and say what you do when a tin falls short.
Run all five again on launch day.
Calculator specification sheet
A single sheet split into three parts. Part one covers the buyer’s inputs, each with its unit, its limits and a sensible default for the commonest room. Part two covers the fields held on each product record, from coverage range to pack sizes and data sheet date. Part three is a test table with columns for room, entered area, estimate, litres actually used and the gap.
Open a blank worksheet to printQuestions owners ask
Should the calculator deduct doors and windows?
Yes, as a count with an opening size you can edit. It stops small rooms being over-ordered. If the buyer types wall area directly, assume she has already deducted them and say so on screen.
Which coverage figure should I trust?
The maker’s data sheet for that product, read at the surface the buyer chose. Treat it as a range. Never present it as a promise. Keep the sheet date on the record.
What if the estimate leaves a buyer a litre short?
The saved inputs show how the figure was reached, so you can compare them with the real room and see whether the buyer or the calculator was out. Offer a matching tin from the same batch if one is left.
Does it work for primer, ceilings and woodwork?
Yes, as separate lines. Each line has its own product, coverage and coats, so a room with walls, a ceiling and a door frame returns three clearly labelled quantities.