How the floor finish calculator works
Every wood floor finish lists a spread rate per coat, but that figure assumes wood that has already been sealed. When you sand a floor back to bare timber, the first coat disappears into the grain and covers noticeably less. The calculator therefore adds a quarter of a coat when you choose a sanded floor, then applies a 10% allowance for the applicator pad, the edges done by brush and the finish left in the bottom of the tray.
Finish = length × width × (coats + 0.25 if bare) ÷ spread rate × 1.10Spread rates used: 10 m² per litre for water-based polyurethane, 12 m² per litre for oil-modified polyurethane and 20 m² per litre for hardwax oil, which is worked thinly into the surface. Imperial sizes are converted with 1 ft² = 0.0929 m², and litres are turned into US gallons (1 gal = 3.785 L).
Worked example
An oak living-room floor of 5 m × 4 m (20 m²) has been sanded to bare wood and will get three coats of water-based polyurethane. One coat needs 20 ÷ 10 = 2.0 L. Three coats plus the extra quarter coat make 3.25 coat-equivalents, so 20 × 3.25 ÷ 10 = 6.5 L, or 7.15 L with the allowance. That is three 2.5 L cans. The US version: a 16 × 13 ft room is 208 ft² (19.3 m²) and takes about 6.9 L, roughly 1.8 gallons, so two gallon cans. The same floor in hardwax oil, two coats, needs only 19.3 × 2.25 ÷ 20 × 1.1 = 2.4 L.
Finishes compared
| Finish | Spread per coat | Usual coats | Recoat / light use |
|---|---|---|---|
| Water-based polyurethane | 9–11 m²/L (370–450 ft²/gal) | 3, sometimes a sealer + 2 | 2–3 h / 24 h |
| Two-component water-based | 9–10 m²/L (370–400 ft²/gal) | 2–3 | 2–4 h / 24 h, full cure about 7 days |
| Oil-modified polyurethane | 11–13 m²/L (450–530 ft²/gal) | 2–3 | 8–12 h / 24–48 h |
| Hardwax oil | 18–24 m²/L (730–980 ft²/gal) | 2 | 8–12 h / 24 h |
Check the times on your product; cold, damp rooms slow every one of them down.
Tips for an even, durable coat
- Vacuum, then tack. Sanding dust left in the gaps is dragged out by the applicator and sets as grit. Vacuum twice and wipe with a barely damp microfibre cloth.
- Keep a wet edge. Work with the grain in lanes about 1 m (3 ft) wide and always finish toward the door you will leave by.
- Abrade between coats. A light pass with a fine screen or 180–220 grit pad gives mechanical grip for the next coat and flattens raised grain after the first water-based coat.
- Do not thicken coats to save time. Heavy coats of oil-based finish stay soft and wrinkle; more thin coats always beat fewer thick ones.
- Allow furniture a week. Most finishes take socks after a day but reach full hardness only after several days. Leave rugs off for two weeks.
- Air the room. Oil-based products release strong solvent vapour; open windows, switch off pilot lights and dispose of oily rags flat to dry, as they can self-heat.
Frequently asked questions
How much polyurethane do I need for 1000 sq ft of floor?
At about 400 ft² per gallon for water-based poly, each coat on 1000 ft² needs 2.5 gallons, so three coats on bare wood with a 10% margin come to roughly 9 gallons.
How many coats of polyurethane on a hardwood floor?
Three coats of water-based or two to three coats of oil-based polyurethane are the usual range; busy kitchens and hallways benefit from a fourth water-based coat.
How many square metres does a litre of floor varnish cover?
Most floor lacquers cover 9 to 12 m² per litre per coat on sealed wood, a little less for the first coat on freshly sanded timber.
How much hardwax oil do I need for a room?
Hardwax oil spreads thinly, around 20 m² per litre per coat, so a 20 m² room with two coats needs about 2.5 litres including the first coat on bare wood.
Can I recoat a floor without sanding to bare wood?
Yes, if the old finish is sound and not waxed: clean, abrade with a fine screen and apply one or two new coats. Check adhesion on a hidden spot first.
Last reviewed: 2026-09-27 · How we calculate