How to calculate how much paint you need

Paint is sold by the gallon, and every gallon covers a set area. Most interior latex paints list a spread rate of 350 to 400 square feet per gallon on smooth, previously painted drywall. To know how many gallons to buy, work out the paintable surface, multiply it by the number of coats, and divide by that spread rate.

The 21 sq ft door deduction matches a standard 3 × 7 ft interior door. The 15 sq ft window deduction matches a typical 3 × 5 ft double-hung window. If you have a sliding patio door or a large picture window, add it as two or three windows.

Worked example: a 12 × 12 ft bedroom

The perimeter is 48 ft. At 8 ft tall the walls measure 384 sq ft. Take off one door (21) and two windows (30) and you have 333 sq ft to paint. Two coats doubles that to 666 sq ft, and at 350 sq ft per gallon you need 1.9 gallons. Buy two gallons. Add the ceiling (144 sq ft) and you move to 2.7 gallons, which is three single gallons or a 5-gallon bucket if you have another room to paint.

Gallons or a 5-gallon bucket?

A 5-gallon bucket usually costs about the same as four single gallons. The calculator compares both and picks the cheaper mix. Once you need four gallons or more, the bucket almost always wins, and every coat comes out the same color because it all came from one batch.

When to add primer

• New, unpainted drywall or fresh joint compound patches.
• Going from a dark color to a light one, or covering red, yellow or deep blue.
• Stains from water, smoke or crayon, which bleed through latex paint.
• Glossy surfaces where new paint needs something to bite into.

Primer spreads a little less than paint, around 300 sq ft per gallon, because bare surfaces soak it in. Paint-and-primer-in-one products work well over sound, similar colors, but they do not replace a real primer on new drywall.

Tips for a more accurate estimate

• Textured walls, like orange peel or knockdown, can use 10–25% more paint. Lower the spread rate to 300.
• Keep a quart of each color for touch-ups. Write the room and date on the lid.
• Trim, doors and baseboards are usually painted in semi-gloss. A quart covers the trim in a typical bedroom.

Frequently asked questions

How to calculate how much tile you need

Measure the floor in feet and multiply length by width to get square feet. Break an L-shaped or irregular room into rectangles, calculate each one, and add them together. Then add overage for cuts, breakage and future repairs. Tile is sold by the box, so divide by the square footage printed on the box and round up.

How much extra tile to buy

Keep the leftover boxes. Tile colors vary between production lots, and a matching replacement can be impossible to find a few years later.

Worked example: 12 × 10 ft kitchen

The floor is 120 sq ft. With 10% overage for a straight lay you order 132 sq ft. A 12 × 24 in tile is 2 sq ft, so that is 66 tiles. At 16 sq ft per box you need 9 boxes (8.25 rounded up).

Thinset and grout

Thinset coverage depends on the trowel. Larger tiles need deeper notches so the back of the tile is fully supported. A 50 lb bag covers about 90–95 sq ft with a 1/4 in square notch, 60–65 sq ft with 1/4 × 3/8 in, and 40–45 sq ft with 1/2 in. Grout depends on the joint width, the tile thickness and the tile size: small tiles have many more joints per square foot, so a mosaic floor can use five times the grout of a large-format floor.

• Use sanded grout for joints 1/8 in and wider, and unsanded for joints under 1/8 in.
• Rectified large-format porcelain is often set with 1/16 in or 1/8 in joints.
• Luxury vinyl plank (LVP) and laminate need no thinset or grout. Use the tile count and box result and ignore the setting materials.

Frequently asked questions

How to calculate concrete in cubic yards

Concrete is ordered by the cubic yard. One cubic yard is 27 cubic feet, a cube 3 ft on each side. Measure length and width in feet and thickness in inches, convert the thickness to feet by dividing by 12, and multiply. Divide cubic feet by 27 to get cubic yards.

Worked example: 10 × 10 ft patio

A 10 × 10 ft slab at 4 in thick is 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 cubic yards. With 10% extra you should plan for 36.7 cu ft (1.36 yd³). In 80 lb bags, which yield 0.60 cu ft each, that is 62 bags weighing close to 2.5 tons.

How many bags of concrete are in a yard?

Bags or a ready-mix truck?

For post holes, a small pad or anything under about one cubic yard, premix bags are simpler and cheaper. Above that, a ready-mix truck saves hours of mixing and gives a stronger, more consistent pour. Most plants charge a short-load fee for orders under 3 to 5 yards, so compare both totals in the calculator before you decide.

Recommended thickness

• 4 in: sidewalks, patios, shed floors, garage floors for cars.
• 5–6 in: driveways, RV pads and floors that carry heavy trucks.
• Footings: at least twice as wide as the wall and set below the frost line. Your local code sets the minimum.

Excavation is rarely perfectly flat. A base that is 1/2 in low across a slab adds about 12% more concrete, which is why the 10% extra matters.

Frequently asked questions

How to calculate bricks and blocks for a wall

Multiply the wall length by its height to get the face area in square feet, subtract doors, windows and other openings, then multiply by the number of units it takes to fill one square foot. That number depends on the unit's face size plus a standard 3/8 in mortar joint.

Masonry units are named by their nominal size, which includes the 3/8 in joint. An "8 × 8 × 16" block actually measures 7-5/8 × 7-5/8 × 15-5/8 in, so with mortar it fills an 8 × 16 in space: 128 sq in, or 1.125 blocks per square foot.

Worked example: a 20 × 4 ft brick garden wall

The wall face is 80 sq ft. At 6.75 modular bricks per square foot that is 540 bricks, or 567 with 5% extra. At about 37 bricks per 80 lb bag of mortar mix you need 16 bags. Build the same wall in 8 × 8 × 16 CMU and you need 95 blocks and 8 bags.

Which mortar type?

• Type N: general above-grade work, veneer and garden walls. Easier to work with.
• Type S: below grade, retaining walls, foundations and anything that carries a load or touches soil.
• Pre-blended mortar mix only needs water. If you mix your own from masonry cement, one 70 lb bag of cement goes with about 2.25 cubic feet of sand.

Mortar coverage varies with the mason. Beginners and walls with many cuts use more mortar, so add a bag or two for a first project.

Frequently asked questions

How to calculate mulch, gravel or soil

Landscape materials are sold by volume. Garden centers sell bags in cubic feet, and landscape yards sell bulk material by the cubic yard. Multiply the area in square feet by the depth in feet (inches ÷ 12) to get cubic feet, then divide by 27 to get cubic yards.

Worked example: a 20 × 4 ft flower bed

The bed covers 80 sq ft. At 3 in deep that is 80 × 0.25 = 20 cu ft. With 5% extra for settling you need 21 cu ft, or 0.78 cubic yards. In standard 2 cu ft bags of mulch that is 11 bags. At about $4.50 a bag the job costs around $50, well under the price of a one-yard delivery.

How deep should it be?

Keep mulch a few inches away from tree trunks and house siding. A "mulch volcano" piled against a trunk holds moisture on the bark and invites rot and insects.

How many bags are in a cubic yard?

Bags or bulk delivery?

Bags are easy to carry and store and cost nothing to deliver, so they win for small beds and touch-ups. Bulk is usually cheaper per yard, but most landscape yards charge a delivery fee and sell in half-yard or one-yard steps. The calculator compares both totals for your job. As a rule of thumb, bulk starts to win at around 2 to 3 cubic yards of mulch, and sooner for gravel and stone, which are heavy to haul in bags.

Weight matters for stone and soil. A cubic yard of gravel weighs about 1.4 tons, well over what most half-ton pickups can carry in one trip. Mulch is light, around 800 lb per yard.

Frequently asked questions