About

Misjudging paint quantity is the most common DIY error in interior finishing. Buy too little and you face color-batch mismatch on the second run. Buy too much and you waste $30 - 80 per unused gallon. This calculator computes net paintable area A_net by subtracting door and window openings from gross wall surface, then divides by your paint's spread rate S m²/L (or ft²/gal) to produce the exact volume required. It factors in number of coats, a configurable waste allowance, and rounds up to whole cans so you leave the store with the right amount.

The formula assumes rectangular rooms with uniform wall height. For rooms with vaulted ceilings, alcoves, or irregular geometry, measure each wall segment separately and sum them manually. Coverage rates in the reference table below are manufacturer mid-range figures measured on smooth, primed surfaces. Rough or porous substrates (bare drywall, textured stucco, unsealed brick) can reduce effective coverage by 15 - 25%. Adjust the waste factor accordingly.

Formulas

The gross wall area for a rectangular room with length L, width W, and height H is computed as the perimeter multiplied by height:

A_walls = 2 × (L + W) × H

If the ceiling is included:

A_ceiling = L × W

Total opening area is the sum of all doors and windows:

A_openings = n_d × w_d × h_d + n_w × w_w × h_w

The net paintable surface is:

A_net = A_walls + A_ceiling − A_openings

Paint volume required, including a waste factor f_w (expressed as a decimal, e.g. 0.10 for 10%):

V = A_net × cS × (1 + f_w)

Where c = number of coats and S = paint spread rate in m²/L or ft²/gal. Total cans needed is rounded up to the nearest whole can:

N_cans = ceil(VV_can)

Where V_can is the volume per can (e.g. 3.78 L for 1 US gallon). Estimated cost:

C_total = N_cans × P_can

Where P_can is the price per can.

Reference Data

Paint Type	Finish	Coverage m²/L	Coverage ft²/gal	Recommended Coats	Dry Time hr	Best For
Latex Interior (Economy)	Flat/Matte	8 - 10	300 - 400	2	1 - 2	Ceilings, low-traffic walls
Latex Interior (Premium)	Flat/Matte	10 - 13	400 - 500	2	1 - 2	Living rooms, bedrooms
Latex Interior	Eggshell	10 - 12	350 - 450	2	2 - 4	Bedrooms, dining rooms
Latex Interior	Satin	10 - 12	350 - 450	2	2 - 4	Hallways, kids' rooms
Latex Interior	Semi-Gloss	10 - 13	400 - 500	2	2 - 4	Kitchens, bathrooms, trim
Latex Interior	High-Gloss	10 - 13	400 - 500	2 - 3	2 - 4	Cabinets, doors, trim
Acrylic Latex	Flat	9 - 12	350 - 450	2	1 - 2	General interior walls
Acrylic Latex	Satin	10 - 12	400 - 450	2	2 - 4	Medium-traffic areas
Alkyd / Oil-Based	Satin	12 - 14	400 - 500	2	6 - 8	Woodwork, high-wear trim
Alkyd / Oil-Based	Semi-Gloss	12 - 14	400 - 500	2	6 - 8	Doors, window frames
Alkyd / Oil-Based	High-Gloss	12 - 15	450 - 550	2 - 3	8 - 16	Furniture, metal surfaces
Primer (Latex)	Flat	9 - 12	300 - 400	1	1	New drywall, patched areas
Primer (Oil-Based)	Flat	8 - 11	300 - 350	1	8	Stain blocking, bare wood
Primer (Shellac)	Flat	9 - 12	350 - 450	1	0.75	Severe stains, smoke damage
Chalk Paint	Matte	8 - 10	300 - 400	2	1	Furniture, decorative
Milk Paint	Matte	7 - 9	250 - 350	2 - 3	0.5	Vintage furniture
Limewash	Matte	5 - 8	200 - 300	3 - 4	24	Brick, stone, plaster
Ceiling Paint	Ultra-Flat	9 - 11	350 - 400	1 - 2	1	Ceilings only
Masonry Paint	Flat	4 - 7	150 - 250	2	4 - 6	Concrete, brick exterior
Epoxy Paint	Semi-Gloss	8 - 11	300 - 400	2	16 - 24	Garage floors, basements
Elastomeric Paint	Flat	2 - 4	75 - 150	2	4 - 6	Cracked stucco, exterior
Anti-Mold Paint	Satin	10 - 12	350 - 450	2	2 - 4	Bathrooms, basements
Texture Paint	Textured	2 - 4	75 - 150	1	24	Feature walls, ceilings
Spray Paint (Interior)	Varies	5 - 8	200 - 300	2 - 3	0.25 - 1	Small items, touch-ups
Whiteboard Paint	Gloss	6 - 8	200 - 300	2 - 3	48 - 72	Offices, playrooms
Magnetic Paint	Flat	2 - 3	50 - 100	3 - 4	4	Feature walls, kids' rooms

Frequently Asked Questions

Rough or heavily textured surfaces (orange peel, knockdown, popcorn) have 20-40% more micro-surface area than smooth drywall. This means paint fills valleys and pores, reducing effective spread rate S significantly. If the reference table lists 12 m²/L for smooth surfaces, expect 8 - 10 m²/L on textured walls. Increase the waste factor to 15 - 20% to compensate.

Primer is necessary on new (unfinished) drywall, over dark-to-light color changes, on stain-prone surfaces, and on bare wood or metal. Primer has its own coverage rate - typically 9 - 12 m²/L for latex primer. Run the calculator once for the primer coat (set coats to 1, select primer coverage) and separately for your topcoat to get accurate totals for both products.

Yes. Transitioning from a dark base to a light topcoat typically requires 3 - 4 coats instead of the standard 2, especially without a tinted primer. A gray-tinted primer reduces topcoat requirements back to 2 coats. Set the coat count accordingly in the calculator for an accurate volume estimate.

The calculator models the room as a simple rectangle. For L-shaped, T-shaped, or irregular floor plans, decompose the room into rectangular segments. Calculate each segment independently (enter its own length, width, and height) and sum the paint volumes. Shared interior walls between segments should not be counted twice - subtract the shared wall area manually or reduce the door/window deduction field.

Rollers are the most efficient at 5 - 10% waste. Brushes lose 5 - 15% to absorption and dripping. Airless sprayers produce significant overspray: expect 20 - 35% waste, especially on trim and corners. HVLP sprayers are better at 10 - 20%. Set the waste factor in the calculator to match your planned application method.

The calculator uses a single average window dimension multiplied by count. If your windows vary significantly, calculate the total window area manually: sum each window's w × h. Then set the window count to 1 and enter dimensions that produce that total area. Alternatively, run the calculator multiple times for different wall sections.