正確な塗料の必要量を計算：ドアと窓を除いた壁面積に、塗り回数と下塗りを掛け合わせ、表面テクスチャによる吸い込み係数とブランド別のm²/Lも考慮します。
| 名前 | 縦 | 横 | 高さ | ドア | 窓 |
|---|
| 部屋 | 壁面積 | 正味面積 |
|---|
1部屋あたりの壁面積: 2 × (縦 + 横) × 高さ。天井を含める場合は 縦 × 横 を加えます。
開口部を差し引く: ドア1枚につき21 sqft、窓1枚につき15 sqft（≈1.95 m²と1.4 m²）。
必要な塗料の量: 総正味面積 × 塗装回数 ÷ 1ガロンあたりの塗布面積。塗布面積は表面の質感によって異なります: 滑らか350、軽いテクスチャ300、重いテクスチャ250 sqft/gal。
下塗り（プライマー）: 全面に1回塗りで約200 sqft/gal。必ず容器単位に切り上げてください。
The calculator works room by room. For each row it takes wall area as 2 × (length + width) × height, optionally adds length × width for the ceiling, then subtracts a flat 21 square feet per door and 15 square feet per window to get net paintable area. Totals across all rooms are multiplied by your coat count and divided by the coverage rate, and the answer is rounded up to whole containers, because you cannot buy two-thirds of a tin.
Surface texture is the coverage rate, and it is the biggest single lever: Smooth gives 350 sqft/gal, Light texture 300 and Heavy texture 250, so a heavily textured wall needs 40 percent more paint than a smooth one for the same area. Coats defaults to 2 and accepts 1 to 5. Turning Primer on adds a separate single coat costed at 200 sqft/gal, reported alongside the topcoat figure rather than mixed into it.
The opening deductions are fixed averages, not measurements: every door counts as 21 sqft and every window as 15 sqft regardless of the real size or the unit system you selected. Trim, doors themselves, closets and soffits are not counted. Metric mode computes in square feet and gallons internally and converts at 3.78541, then rounds to whole litres — which will not line up with the 2.5 and 5 litre tins actually sold, so round to real tin sizes yourself.
With 8 foot ceilings that is 384 sqft of wall. Deduct one door and one window and you get 348 sqft net; two coats at the smooth 350 sqft/gal rate works out to 1.99 gallons, which the tool rounds up to 2 gallons.
No. Each room row has Doors and Windows count fields. Enter how many of each and the tool removes 21 sqft per door and 15 sqft per window automatically. If a room has an oversized picture window, enter it as two windows to approximate the area.
Only if you set Include ceiling to Yes, which adds length × width to every room's area. It is off by default, and it uses the same texture coverage rate as the walls, so adjust the texture setting if your ceiling is a different finish.
Keep coats at 2 and switch Primer on. Primer is costed separately as one coat at 200 sqft/gal, which is the standard approach for going dark to light, over patched drywall, or onto bare or glossy surfaces where topcoat alone will not sit evenly.
Because the result is rounded up to whole containers. Adding a few square feet can push 1.98 gallons to 2.01 and force a third tin. The per-room table below the result shows the underlying gross and net areas so you can see what actually moved.
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