Guides
Construction Math Guides
Plain-language explainers for the numbers behind a materials order: area, volume, weight, bags, blocks, bricks, mortar, tiles, paint, flooring, carpet, stairs, bundles, studs and board feet. Each guide works through real examples and links to the calculator that does the same math for your measurements.
Area and volume
The conversions almost every estimate starts with.
How to calculate square feet
Measure a room, a house or a lot, and split irregular shapes into rectangles, triangles and circles you can add up.
How many square feet are in an acre?
An acre is 43,560 square feet. See where the number comes from and what it means for lot sizes.
Square feet to square yards
Divide square feet by 9. A conversion table and carpet examples where roll width sets what you pay for.
How many cubic feet are in a cubic yard?
There are 27, because a yard is 3 feet on each side. Quick conversions for concrete, gravel and soil.
How to calculate cubic yards
Length × width × depth in feet, divided by 27. Five worked examples, from rectangles to circles.
How many cubic yards are in a ton?
There is no single conversion. Typical tons per cubic yard for gravel, sand, soil and concrete.
Average square footage of a house
New single-family houses completed in 2025 had a median of 2,142 sq ft, per the U.S. Census Bureau. Trend, regions and how to measure yours.
Concrete and masonry
Bags, pallets, posts, blocks, bricks and mortar.
How many bags of concrete do I need?
Turn a slab, a post hole or a cubic yard into bags of 80, 60, 50 or 40 lb, using the yield printed on the bag.
How much concrete do you need for a fence post?
A 4×4 post in a 3 ft hole takes about 2.6 bags of 80 lb mix and a 6×6 about 6.4. Table by fence height.
How many concrete blocks do I need?
An 8×8×16 block covers 8/9 sq ft, so a wall takes 1.125 blocks per sq ft. Mortar and core fill included.
How many bricks per square foot?
Modular brick with 3/8 in joints takes 6.75 per sq ft. Wall examples, the joint effect, 5% waste and mortar bags.
How much mortar do I need?
About 5.5 cubic feet of mortar per 100 sq ft of modular brick wall. Bags per 100, 500 and 1,000 bricks or blocks, and mortar waste.
How many bags of concrete are on a pallet?
A pallet of QUIKRETE Concrete Mix #1101 holds 42 bags of 80 lb, 56 of 60 lb or 80 of 40 lb. Yards and weight per pallet.
Landscaping
Mulch, gravel, topsoil and what to order by the yard or the ton.
How many bags of mulch are in a cubic yard?
A cubic yard is 13.5 bags of 2 cu ft, or 9 bags of 3 cu ft. Coverage by depth included.
How much gravel do I need?
Gravel in cubic yards and tons from area and depth, with typical weights by stone and a compaction allowance.
How much topsoil do I need?
Cubic yards = area × depth in inches ÷ 324. Depths for lawns and raised beds, weight and bags.
Interior
Sheets, tiles, flooring, carpet, paint, trim and what to add for waste.
How many sheets of drywall do I need?
Board area plus about 10% waste, divided by 32, 40 or 48 sq ft per sheet. Tape, mud and screws included.
How many tiles do I need?
Floor area divided by one tile plus its joint, plus waste, rounded up. Boxes, thinset and grout included.
How many square feet does a gallon of paint cover?
About 350 to 450 sq ft per coat, depending on the product. Coverage, primer and a room example.
How much baseboard do I need?
Perimeter minus doorways, plus 10% waste, divided by the piece length. Examples for a room and a whole floor.
How much flooring do I need?
Room area plus 5 to 15% waste, divided by the square feet on one box, rounded up. Waste by layout and a 12 × 15 ft example.
How to measure for carpet
Measure a room wall to wall, add about 10% and divide by 9 for square yards. Why roll widths of 12, 13 ft 6 in and 15 ft change the order.
How to measure stairs for carpet
Tread, riser and a nose allowance for each step, times the stair width, plus about 10%. A worked 14-riser example and a table by step count.
Roofing and exterior
From a footprint to a shingle order, and from a joist layout to a stack of deck boards.
How to calculate roof square footage
Use the pitch factor to turn a footprint into real roof area, then into roofing squares.
How many shingles are in a bundle?
Bundles per square, coverage per bundle and why some lines need four bundles per square instead of three.
How to calculate roof pitch
Pitch is rise per 12 inches of run. Three safe ways to measure it and a chart for degrees and percent.
How many deck boards do I need?
Rows come from deck width divided by board width plus gap. Waste for diagonal layouts and fastener counts.
Lumber and length
Board feet, linear feet, studs, stair layout and how lumber is priced.
How to calculate board feet
Thickness × width × length in feet ÷ 12. Nominal vs actual size, a 2×4×8 example and a lumber size table.
How to calculate linear feet
Square feet × 12 ÷ board width in inches. A table of linear feet per 100 sq ft, for baseboard, planks and decking.
How many studs do I need?
Wall length in inches ÷ spacing, rounded up, plus 1. A table at 16 and 24 in on center, with extras for corners and openings.
Stair rise and run: standard riser height and tread depth
The 2015 and 2018 IRC text caps risers at 7 3/4 in with a 10 in minimum tread. Risers, treads and run from the total rise, with tables.
How to lay out stair stringers
Riser height and treads from the total rise, a framing square stepped along the board and the diagonal length. Layout geometry only.
How the guides are written
Numbers come from named sources such as NIST Handbook 44 and manufacturer data sheets, or they are plain arithmetic shown step by step. Allowances are labeled as conventions you can change. Read the full methodology.