Understanding your result
The headline is the siding paint for the coats you chose, with the allowance, and the sub-line gives the cans for the siding and for the trim. The three figures show the net siding area, the siding cans and the trim paint.
The breakdown goes from the walls (perimeter times height) through the gables, the openings and the net area, applies the siding factor to get the effective area, and gives the siding paint with its cans. If the trim is a second colour, it then shows the trim area as a 6 in band and the trim paint with its cans. Two products, two can lists, two lines on the shopping list.
Sheds are small enough that the rounding is most of the answer: 1.85 gallons buys two gallons, and the trim’s 1.18 quarts buys two quarts. The exterior house paint calculator is the same arithmetic for a house, with more gables and storeys; the exterior trim calculator measures trim piece by piece if the shed’s trim is wider than a 6 in band; and the garage door calculator handles a workshop’s roll-up door, which is counted here as an opening.
How we calculate this
The walls and gables are the same geometry as a house: a rectangle wrapped around the building plus a triangle at each gable end, which on a shed spans the width. The openings use the standard 20 sq ft door and 15 sq ft window from the standard sizes page; a double shed door is two doors. The siding factors are the ones the siding guide explains for houses, with grooved T1-11 in the rough class because its grooves add surface and its face drinks.
The trim is a 6 in wide band: the fascia along the eaves and rakes (the perimeter), the four corner boards (four times the wall height), and the casing around each door (18 ft of perimeter) and window (12 ft). On a 10 × 8 shed that is 47 sq ft, and the 10% allowance is the brush loss on narrow boards. The coverage is the exterior paint’s figure for both, from the coverage rates table; trim paint is often a different sheen of the same line. The exterior measuring guide shows how to measure a gable height without a ladder.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Coverage | 350 sq ft per gallon | Exterior acrylic data sheets (Sherwin-Williams, Benjamin Moore, Behr): 250–400 by surface; 350 smooth |
| Siding factors | smooth 1.0, lap 1.1, rough / grooved T1-11 1.3 | As the exterior house calculator; the low end of manufacturers’ rough-surface coverage as a multiplier |
| Openings | door 20 sq ft, window 15 | Standard sizes page |
| Trim band | 6 in wide; door casing 18 ft, window casing 12 ft of perimeter | Site estimate for typical shed trim |
| Coats | 2 | Manufacturers’ recommendation for a colour change or new siding |
| Allowance | 5% siding, 10% trim | Roller and brush loss |
Assumptions last reviewed October 7, 2026.
The calculator does not model primer on bare wood (the primer calculator takes the net siding area in its measured mode), the roof, the door itself if it is a garage-style door, or the interior. A metal shed is painted with a direct-to-metal enamel at the smooth factor, after washing and a scuff.
Two worked examples
A garden shed in grooved T1-11
10 × 8 ft, 7 ft walls, two gables 2.5 ft high, one door and one window, rough siding, trim in a second colour, two coats.
- Walls: 2 × (10 + 8) × 7 = 252 sq ft; gables: 2 × ½ × 8 × 2.5 = 20 sq ft
- Net siding: 272 − 35 = 237 sq ft; effective: 237 × 1.3 = 308 sq ft
- Siding paint: 308 ÷ 350 × 2 × 1.05 = 1.85 gallons, buy 2 gallons
- Trim: (36 + 28 + 18 + 12) × 0.5 = 47 sq ft; trim paint 47 ÷ 350 × 2 × 1.1 = 1.18 quarts, buy 2 quarts
Two gallons of the body colour and two quarts of the trim colour. A lean-to of the same size in smooth siding, no gables, trim the same colour, one coat as a refresh, is 232 sq ft and 2.78 quarts: three quarts.
A 24 × 12 workshop in lap siding
8 ft walls, two 4 ft gables, two doors and two windows, lap siding, two coats, trim separate.
- Walls: 2 × (24 + 12) × 8 = 576 sq ft; gables: 2 × ½ × 12 × 4 = 48 sq ft
- Net siding: 624 − 70 = 554 sq ft; effective: 554 × 1.1 = 609 sq ft
- Siding paint: 609 ÷ 350 × 2 × 1.05 = 3.66 gallons, buy 3 gallons + 3 quarts
- Trim: 82 sq ft; trim paint 2.06 quarts, buy 3 quarts
Four gallons of body colour in practice, and three quarts of trim. If the workshop has a roll-up door, count it as three doors in the openings and run the garage door calculator for the door itself.
Where to find your inputs
Length, width and height. Outside dimensions along the base, and the wall height to the eave on the low side of a lean-to or either side of a gable roof.
Gables. Two on a gable roof, spanning the width; measure the height from the eave to the peak, or estimate it from the pitch (a 4:12 pitch on an 8 ft wide shed is 1.33 ft). Zero for a flat or lean-to roof.
Doors and windows. Count openings; a double door is two.
Siding. Grooved T1-11 and rough-sawn boards are rough; smooth plywood, metal and vinyl are smooth; lap is lap.
Common mistakes
- Rolling grooved T1-11 without a brush. The grooves stay bare; brush them first or back-brush a sprayer.
- Skipping primer on bare plywood. The edges soak and delaminate; prime, especially the bottom edge.
- Forgetting the gables. Twenty square feet on a small shed, fifty on a workshop.
- One coat on new siding. It flashes and fades in a year.
- Painting the roof. Shingle and metal shed roofs are not painted.
- Buying trim paint by the gallon. Two or three quarts do a shed.
Questions people ask
- How much paint does a shed take?
- A 10 × 8 ft shed with 7 ft walls, two small gables, a door and a window in grooved T1-11 has about 237 sq ft of net siding, 308 sq ft after the rough-surface factor. Two coats at 350 sq ft per gallon with a 5% allowance is 1.85 gallons, so two gallons, plus two quarts of trim paint.
- What is the siding factor for T1-11?
- Grooved T1-11 and rough-sawn boards take about 30% more paint than a smooth panel, because the grooves and the texture add surface and soak up the first coat. Smooth T1-11, metal and vinyl are 1.0; lap siding 1.1. The factor multiplies the net siding area and is shown in the result.
- How is the trim estimated?
- As a 6 in wide band along the eaves, down the four corners and around each door and window, about 47 sq ft on a 10 × 8 shed. At two coats that is just over a quart, so two quarts of the trim colour. Untick the trim box if the trim is the same colour as the siding and it is folded into the siding figure.
- Do I need to prime a new shed?
- Yes, if the siding is bare wood, with an exterior wood primer, and the quantity is about the same as one coat of paint. Pre-primed T1-11 and previously painted sheds that are sound can be painted directly. The primer calculator gives the figure for the net siding area.
- Paint or stain for a shed?
- Paint for T1-11 and smooth siding, where a film protects the plywood edges. Solid stain for rough-sawn boards and board-and-batten, where it weathers better than paint and does not peel. The fence stain calculator has the stain coverages if you go that way.
- Is the roof included?
- No. Shingle and metal shed roofs are not painted. A flat shed roof with a coating is the roof coating calculator's job, and the trim figure here covers the fascia along the roof edge.