この計算機の使い方
What this calculator is for, and when it will not fit your job
This calculator answers one practical question: how much paint should you buy so you can finish the job without a second trip to the store. You feed it the wall perimeter of a room, the wall height, the total area of doors and windows you will not paint, and the number of coats you plan, and it returns the area to paint, the whole cans to buy, and an estimated cost. It is built for planning a repaint of one room, a hallway, or a whole floor before you order anything. The unit toggle switches the whole estimate between feet and gallons and meters and liters, so it works whether your tape measure is marked in inches or centimeters. And because every calculation runs locally in your browser, you can play with the numbers, add a coat, raise the waste allowance, and see how the can count changes before you spend a dollar.
Know its limits before you trust the number. The calculator models rectangular walls with a uniform height, so cathedral ceilings, sloped walls, and rooms with lots of alcoves are approximated, not measured exactly. It estimates walls only: ceilings, trim, baseboards, and doors are separate surfaces with their own areas and usually their own paint. The coverage rate, 350 square feet per US gallon per coat, is the contractor planning rule, not a property of your specific wall: texture, sheen, primer, and application technique all change real consumption, which is why the results always round up to a whole can and why a small waste allowance is the safer habit. Think of the estimate as your starting point for budgeting, not a guarantee of how much you will pour out of the cans. A painter who trusts a rule of thumb without checking the room ends up back at the store mid-job, and that is the trip this calculator is meant to save.
How to fill in the six fields: perimeter, height, openings, coats, waste, and price
Start with the unit toggle: Feet/gallons or Meters/liters, and keep every measurement in that system. Wall perimeter is the total length of all walls you plan to paint, walked around the room at floor level and added up. Include alcoves and bay-window returns, and leave out walls you are not painting, like a closet behind a door you will keep closed. Wall height is floor to ceiling, or floor to the trim line if a chair rail or wainscot marks the upper edge of the painted area. Doors plus windows is the total area you will not paint: measure each opening, height by width, and add them together, in square feet for imperial or square meters for metric. The classic mistake is mixing systems, entering a 3.5-meter wall into a feet calculation, which turns a real room into a fantasy one. When in doubt, re-measure rather than estimate, because the whole result scales linearly with the perimeter.
Coats is how many layers the walls will get, from 1 to 10, and primer counts as a coat: a new wall that needs primer plus two finish coats is entered as 3. Waste percent is the allowance for cut-ins around trim, touch-ups, drips, and paint left in the roller; 5 to 10 percent is the common range, and 0 means no allowance at all. Price per can is what one gallon, or one liter in metric mode, costs where you buy it, and it affects only the estimated cost: the can count stays the same whether you type 30 or 60, so leave it at 0 if you only care about how much paint to buy. Every field must be a number, coats must be at least 1, and waste percent can go up to 100, though anything above 20 usually means you are paying to pour paint down the roller tray. The calculator flags impossible entries with a clear message rather than guessing.
Reading the results: net area, paint area, cans to buy, and cost
The results panel shows four numbers that build on each other. Net wall area is the total wall surface after doors and windows are subtracted, the area the paint will actually touch. Area to paint multiplies that by your coats and adds the waste allowance, so it is always larger than the net area: two coats do not paint twice as fast, they double the paint you need. Both areas are rounded to whole square feet or square meters for readability, and the bars beside them make the relationship visible at a glance. A room with 600 square feet of walls, 21 square feet of openings, and two coats comes out to 579 net and 1,158 to paint. When the jump between the two numbers looks bigger than you expected, recheck the coats and the waste percent before you blame the room, because those two fields move the area to paint far more than a missed inch of perimeter.
Cans to buy is the number that matters at the checkout: the exact amount of paint, divided by the coverage of one can, rounded up to the next whole can, because stores do not sell a third of a gallon. That rounding is deliberate: a partially used can stays useful for touch-ups, and buying one extra can is cheaper than a second trip for a half-pint. Estimated cost is the cans to buy times the price per can you entered, formatted as money. If the estimate feels high, revisit the waste allowance and the coats before the measurements, since those are the dials you control. And keep the coverage rate honest in your head: it is a planning number, not a property of your wall, so on heavily textured or previously unsealed surfaces real consumption runs higher. When the result sits close to a can boundary, like 3.9 gallons, rounding up in your mind as well is the habit that keeps a job from stalling halfway.
結果はどうやって計算されるの？
寝室の塗り直し：外周 60 ft、高さ 10 ft
寝室の壁の外周は合計60フィート、天井高は10フィートなので、壁の総面積は600平方フィートです。窓2つとドアの開口部は合計約21平方フィートで、残りは579平方フィートです。2回塗りでロス率ゼロなら、塗装面積は1,158平方フィート。1ガロンあたり350平方フィートなら約3.31ガロンなので、4ガロンを購入すべきです。
入力と出力の例 | 入力 | 値 |
| 壁の外周 | 60 |
| 壁の高さ | 10 |
| ドア・窓 | 21 |
| 塗り回数 | 2 |
| ロス率 % | 0 |
| 1缶の価格 | 40 |
| 結果 | 正味 579 sq ft・塗装面積 1,158 sq ft・購入 4ガロン・約 $160 |
Repainting a living room and hallway: 88 ft perimeter, 9 ft height, 10% waste
This example is the one that trips up most DIY painters: a living room and hallway where the first estimate came up short, because nobody wrote down the openings or remembered the second coat. Walked at floor level, the walls total 88 feet of perimeter, and they run 9 feet from floor to ceiling, so the gross wall area is 88 × 9 = 792 square feet. Two windows, a front door, and a hallway door take 48 square feet of openings out of the picture, leaving 744 square feet of wall that will actually see paint. You plan two coats, because a deep color over a light one does not cover in a single pass. Without any allowance, two coats mean 1,488 square feet, which at 350 square feet per gallon is about 4.25 gallons, an awkward number that leaves you guessing whether to buy four cans or five. But you will also cut in around the trim, touch up the corners, and spill a little, so you add a 10 percent waste allowance: 744 × 2 × 1.10 is 1,637 square feet, about 4.68 gallons, and since stores do not sell fractions of a gallon, the calculator rounds up to 5 cans. At $55 per gallon the estimate is $275, about $30 more than the raw gallon count suggests, and that is the price of finishing in one trip. Planning that slack is like a painter measuring twice before cutting the brush edge: the extra minute up front costs less than a second visit to the store. Keep the fifth can sealed and it will still do touch-ups a year later.
入力と出力の例 | 入力 | 値 |
| 壁の外周 | 88 |
| 壁の高さ | 9 |
| ドア・窓 | 48 |
| 塗り回数 | 2 |
| ロス率 % | 10 |
| 1缶の価格 | 55 |
| 結果 | 744 sq ft net · 1,637 sq ft to paint · buy 5 gallons · ≈ $275 |
Fresh-plastered kitchen in metric: 14 m perimeter, 3 coats (primer plus two finish)
A freshly plastered kitchen is a special case, because the walls have never been sealed and the numbers behave differently. The room measures 14 meters around at floor level with 2.5-meter walls, so the gross area is 14 × 2.5 = 35 square meters. A window of about 1.6 square meters and a door of 2.4 square meters take 4 square meters out, leaving 31 square meters of bare plaster. Bare plaster needs a primer coat before finish paint adheres evenly, and you want two finish coats on top; in this calculator primer counts as a coat, so you enter 3 coats with no waste allowance, and the area to paint becomes 31 × 3 = 93 square meters. In metric mode the same coverage rate is expressed in liters, about 8.6 square meters per liter, so 93 divided by 8.6 is about 10.8 liters, and the calculator rounds up to the next whole can: 11 liters. At $32 per liter that is $352 for the paint alone, which sounds like a lot for one kitchen until you remember that a third of it is primer. Buying the full 11 liters is like a tailor adding up the fabric before cutting: the primer eats a third of the paint, and stopping mid-job to run to the store costs more than the extra liter. You can also enter 2 coats instead of 3 and watch the estimate drop from 11 liters to 8, which is a real decision worth making on paper before you stand in front of the shelf.
入力と出力の例 | 入力 | 値 |
| 壁の外周 | 14 |
| 壁の高さ | 2.5 |
| ドア・窓 | 4 |
| 塗り回数 | 3 |
| ロス率 % | 0 |
| 1缶の価格 | 32 |
| 結果 | 31 m² net · 93 m² to paint · buy 11 liters · ≈ $352 |
よくある質問
カバー率には下塗りが含まれますか？
1ガロンあたり350平方フィートというカバー率は、下地処理済みの標準的な表面への仕上げ塗り1回分です。下塗りは別の1回です。新しい面や大胆な色変更を塗る場合は、塗り回数に下塗りを入れてください（例：下塗り＋仕上げ2回で3回）。
なぜロス率が必要なのですか？
トリム周りのカットイン、質感のある面への塗装、タッチアップ、こぼれはすべて余分なペンキを消費します。ほとんどの塗装業者は計算面積の5〜10%を上乗せし、次の整数の缶を買うことで通常はまかなえます。
天井にも同じ推定値を使えますか？
いいえ。この計算ツールは外周と高さで壁を測ります。天井の場合は床の寸法を使ってください。長さ × 幅が天井の面積になり、同じカバー率で割ります。
Why does the result say to buy 4 gallons when the exact amount is 3.31?
Because paint is sold in whole cans and the calculator rounds up: you cannot buy 0.31 of a gallon, and rounding down would leave you short. The leftover from the fourth can is not waste: it is the same color as your wall, and a sealed can remains usable for touch-ups for a long time. The cost line follows the whole can count, so you budget for what you will actually pay at the register. If the exact figure lands just under a whole can, that is a good moment to review the waste allowance before buying.
My room is not a rectangle, with an alcove and a bay window. How do I measure the perimeter?
Measure the real total length of wall you will paint: run the tape along the baseboard of every wall, including the alcove returns and the angled bay-window walls, and add the segments. That total is the perimeter to enter. Doorways you will not paint stay in the count, because you subtract them with the openings field anyway. For walls that are not vertical, like stairwells, the calculator assumes a uniform height, so use the average height where the slope is gentle. For irregular rooms the result stays an approximation, so round up when the number lands near a can boundary.
What should I put for price per can when the tool is in meters and liters?
The price of one can in the unit system you are using: in Feet/gallons mode, the price of one US gallon; in Meters/liters mode, the price of one liter. The field only feeds the estimated cost, so the can count is correct even if you leave the price blank or set it to 0. Note that real stores sell paint in 1, 2.5, and 5 liter cans rather than exact liters, so for an accurate budget, multiply the cans you actually plan to buy by their real prices.