Three numbers, in this order
Every material in the planner goes through the same three steps:
- Exact amount. What the surfaces need on paper: liters or gallons of paint, square meters or square feet of flooring, meters or feet of baseboard.
- With allowance. The exact amount plus a percentage for waste: cut ends, offcuts that are too short, damaged pieces, rollers and trays, touch-ups. With 10%, the amount is multiplied by 1.1.
- Packages. The amount with allowance divided by the package size, then rounded up to a whole number. Cans, boxes, rolls and lengths are only sold whole.
What you buy is the number of packages × the package size. The difference between that and the amount with allowance is the leftover.
The allowance is added before rounding, not after. Rounding to whole boxes first and then adding 10% to the box count would round twice and usually buy more than you need.
Worked example: paint
A room 5 m × 3.6 m with 2.4 m walls, a door of 0.8 m × 2.0 m and a window of 1.4 m × 1.2 m, painted with two coats of a paint with an example coverage of 10 m²/L (a cautious figure: an “up to 12 m²/L” label figure is a best case, see paint coverage and coats):
- Walls: 2 × (5 + 3.6) = 17.2 m × 2.4 = 41.28 m², minus 1.6 m² and 1.68 m² of openings = 38 m²
- Exact amount: 38 × 2 ÷ 10 = 7.6 L
- With 10% allowance: 7.6 × 1.1 = 8.36 L
- In 2.5 L cans: 8.36 ÷ 2.5 = 3.34, rounded up to 4 cans
- Bought: 4 × 2.5 = 10 L
- Leftover: 10 − 8.36 = 1.64 L
The leftover is on top of the allowance. The allowance is meant to be used up by cuts, rollers and touch-ups; anything it does not use adds to the leftover.
Worked example: flooring, and the boundary
A floor of 20 m² with boxes of 2.2 m² and a 10% allowance:
- With allowance: 20 × 1.1 = 22 m²
- Boxes: 22 ÷ 2.2 = 10, exactly, so 10 boxes
- Leftover: 0 m² beyond the allowance
Now the amount with allowance is 22.01 m², just 0.01 m² more:
- Boxes: 22.01 ÷ 2.2 = 10.005, rounded up to 11 boxes
- Bought: 11 × 2.2 = 24.2 m²
- Leftover: 24.2 − 22.01 = 2.19 m²
A tiny change in the measurement adds a whole box. If you are close to a boundary, remember that the allowance is already a margin: 22 m² of flooring for a 20 m² floor leaves 2 m² for cuts. Lower the allowance to stay at 10 boxes only if your layout needs fewer cuts.
An amount that is exactly a whole number of boxes, like the 22 m² above, stays at that number of boxes; it is not pushed up by one.
Only the same material entry is added together
Before rounding, the planner adds up every room and surface assigned to the same material entry (one product, color, finish and package size). Two entries that look identical are flagged, not merged. Two rooms that each need 1.2 L from the same paint entry need 2.4 L together, which is one 2.5 L can (with no allowance). If the two rooms are painted in different colors, each needs its own can, so you buy two.
Primer, wall paint and ceiling paint are separate materials; flooring and underlay are separate too. They are never combined, because the leftover of one cannot be used for the other.
Smaller sizes and mixed sizes
Sometimes a smaller size would leave less over: a paint job that needs about 2.03 gallons (2.025) is 3 one-gallon cans, but 2 gallons plus a quart (2.25 gallons) would also cover it. Whether that is cheaper depends on the prices at your store. Where size comparison exists, you can enter up to two more sizes of the same product with their prices: the planner then plans the purchase as the lowest total among the sizes and prices you entered, shows what each size alone would cost and puts one line per size on the shopping list. It works for:
- Paint, primer and floor finish (varnish or oil) in the paint calculator, the room refresh planner and the exterior paint calculator (“Compare other can sizes of this paint”, “of this primer” or “of this finish” on the card).
- Stain, sealer or oil in the deck, fence and patio stain calculator. Cleaners, brighteners, strippers and jointing sand use one size.
- Seed and feed in the lawn seed, turf and feed calculator. Turf rolls use one size.
Flooring, tiles, wallpaper and the other materials are counted in the one size you enter. The mulch, soil and gravel calculator works a little differently: it shows each bag or bulk option you enter side by side, each rounded up on its own, so you can compare them, but it does not combine two sizes in one purchase.
Reading the leftover
A large leftover usually means one of two things:
- The package is large compared with the job (a 5 L can for a small room).
- The amount is just above a whole number of packages, as in the 22.01 m² example.
Leftover paint is useful for touch-ups. Leftover flooring is useful for repairs, because the same pattern and batch may be hard to find later (see the flooring calculator on lot numbers). Return rules for unopened packages differ between stores; check their policy before you rely on it.
Already bought
If you enter how many packages you have already bought, the planner compares that with the current requirement and shows what is still to buy, or how many packages you have more than you now need. The rounding itself does not change: it always starts from the exact amount.
See the rounding in action in the paint calculator, the flooring calculator and the room refresh planner. For the measurements that go in, read how to measure a room.