Live openings net and buy-list shelf - so gross wall area never pretends to be the cans you should purchase.
Interior wall faces with typical door/window sizes. Not trim, cabinets, or spray-loss engineering.
By Ahmet C. Toplutaş·Site owner & editor · Home & DIY tools
Last updated May 21, 2026 · Reviewed May 21, 2026 · Planning estimates only - not tax, medical, or investment advice
01
Measuring length × height × walls gives a gross face. That number is not what you should fund at the paint counter. Doors and windows are not painted with wall paint in the same pass, coats multiply the job, and rollers waste product the flat formula never sees.
This page’s framing is gross wall ≠ paint to buy. The panel walks an openings net path, then a buy-list shelf that rounds exact liters into whole cans - the difference between a spreadsheet and a shopping cart.
Decision framing: “What net face am I coating, at what coverage, with how much buffer - and which can size do I actually purchase?” - not “room area ÷ 400.”
02
A bedroom with one door and one window can lose a meaningful slice of gross wall area. A living room with a patio door and several windows loses more. The openings step uses typical sizes (~20 ft² door, ~15 ft² window) so counts alone get you close; measure when the opening is unusual.
If openings ever exceed gross area in the inputs, the panel clamps net face to zero rather than inventing negative paint - a signal to re-check counts.
03
Manufacturers print coverage as area per gallon or liter under ideal conditions. Dark colors over light, textured drywall, bare plaster, and cheap paint all reduce real spread rate. The panel accepts sq ft/gal or m²/L so you can match the label language on the can in your hand.
When unsure, lower the coverage number (fewer square feet per gallon). That pushes volume up and is usually cheaper than a mid-project emergency store run in a mismatched batch.
04
Two coats means funding roughly twice the net face in this sketch. Color changes and bold accents often need a third. Waste (default 10%) covers cutting-in, roller saturation, touch-ups, and the reality that not every milliliter reaches the wall as a perfect film.
Porous surfaces and drastic color shifts may deserve 15%+. The waste field is intentional - hide it and DIY math becomes optimism.
05
Exact liters are useful for comparing products. Stores sell sealed cans. The shelf shows how many 1 L, 5 L, 1 gal, and 5 gal containers cover the exact need after ceil rounding. Pick the size that matches how your local aisle is stocked - and prefer same-batch gallons when color match matters.
Buying two 1-gal cans instead of a fractional “1.4 gal” is not wasteful padding - it is how packaging works. Leftover sealed paint is a future touch-up asset if stored well.
06
Example A · Bedroom. Preset: 12×8 ft, 4 walls, 1 door, 1 window, 2 coats, 10% waste, 350 ft²/gal.
Watch openings net shrink the gross face, then coats and waste raise funded area. Read the shelf for 1-gal vs 5 L options - choose what your store actually carries.
Example B · Living room. Preset: more openings. Compare openings % of gross to the bedroom - the subtract step matters more as glass and doors increase.
Example C · Accent wall. Preset: 1 wall, 3 coats, lower coverage, 15% waste. Bold color on one face still burns surprising volume because coats × poorer coverage compound.
Drop coverage from 350 to 300 ft²/gal and watch gallons rise. That is the honest “deep color” tax before you open a brush.
Example D · Metric room. Preset: meters + m²/L. Same openings net story - only the coverage unit language changes to match metric cans.
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Stop here for interior wall faces with simple rectangular openings. Hand off for floors, structural stairs, or exterior multi-surface takeoffs that need different coverage rules.
10
Gross wall area is length × height × walls before openings. Paint to buy is what remains after subtracting doors/windows, multiplying by coats, adding a waste buffer, and rounding up to whole cans. Skipping openings or waste is how DIY estimates come up short mid-project.
It shows exact volume needed, then how many 1 L / 5 L / 1 gal / 5 gal containers to purchase after rounding up. Exact liters are planning math; the shelf is what you put in the cart.
Use the number on your paint can. Typical interior latex is often ~350–400 sq ft per U.S. gallon (about 8–10 m²/L), but primers, dark colors, textured walls, and cheap paint can cover less. When unsure, use a lower coverage number so you over-buy slightly.
Cutting in, roller loss, touch-ups, and surface porosity eat paint that the flat-area formula ignores. A 10% buffer is a common planning default; porous or color-change jobs may need more.
In this sketch, yes - coats multiply the net paintable area. Some second coats spread farther on a sealed first coat; still plan for the labeled coat count rather than hoping one coat hides everything.
Presets use typical opening sizes you can edit. Measure your actual openings when accuracy matters - a double door or picture window changes the net face more than people expect.
No. Values hydrate from the URL on load, but the address bar updates only when you use Share or Copy. That keeps live editing calm and share links intentional.
LIVE PAINT ESTIMATE
Gross ≠ buy ↓Room presets
Updates live. Gross wall ≠ paint to buy. Opening sizes are typical defaults - edit counts to match the room.
Paint to buy (with waste)
2.19 gal
8.3 L · net face 349 ft² · after openings & coats
OPENINGS NET · GROSS ≠ BUY
Step 1 is what people measure. Step 4 is what coverage should fund. The gap is why “room length × height × walls” alone under-buys.
BUY-LIST SHELF · EXACT → ROUND UP
1 L cans
9
5 L cans
2
1 gal cans
3
5 gal pails
1
Exact need: 8.3 L / 2.19 gal. Shelf counts are ceil’d so you leave the store with enough sealed product - not a fractional can.
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