Construction calculator

What is a construction calculator?

A construction calculator works in the units a tape measure shows, feet, inches and fractions or millimetres, so you can add and split real measurements without converting them first. Most also carry the sums builders do every day: area, concrete volume, roof pitch, stair risers and the diagonal that proves a frame is square. This page does both in your browser, free and with no sign-up.

1. Pick Feet and inches or Metric. Switching converts the lengths already entered.
2. Enter the first length (A). In feet and inches, type the feet, the inches, and pick the fraction, or type it in the inches box as 6 1/2.
3. Pick + or − and enter a second length, or × or ÷ and enter a plain number.
4. Read the answer in feet-inches, inches, decimal feet, mm and m. Use answer as A carries it into the next sum.

How to add and subtract feet and inches

Turn both lengths into inches, add or subtract, then turn the inches back into feet, inches and a fraction. Fractions add like any fraction, once they share a denominator.

1. 12' 6 1/2" is 12 × 12 + 6.5 = 150.5".
2. 3' 4 3/8" is 3 × 12 + 4.375 = 40.375".
3. Add: 150.5 + 40.375 = 190.875", which is 190 7/8".
4. Back to feet: 190.875 ÷ 12 = 15 feet and 10.875" over, so the answer is 15' 10 7/8".
5. The same length is 15.9063 ft in decimal feet and 190.875 × 25.4 = 4,848.2 mm.

Subtracting works the same way. 40.375 − 150.5 = −110.125", so 3' 4 3/8" − 12' 6 1/2" is −9' 2 1/8": the minus sign tells you the second length was the longer one. The calculator keeps every sum exact in inches and rounds only the answer it shows.

How to multiply or divide a measurement

Multiply or divide the length in inches by a plain number, then convert back. Dividing is how you split a length into equal parts; multiplying adds up repeated pieces.

1. Split 8' 1 1/2" into three: 8 × 12 + 1.5 = 97.5", and 97.5 ÷ 3 = 32.5", which is 2' 8 1/2" (825.5 mm).
2. Seven 16" spaces: 16 × 7 = 112", which is 9' 4".
3. A split that doesn't come out even: 100" ÷ 3 = 33.333". To the nearest 1/16" that is 2' 9 5/16", 0.0208" short of exact; at 1/32" it is 2' 9 11/32". The calculator says how far each rounding moved the answer.

To lay out equal gaps between posts or balusters rather than split a length, see how to divide a length into equal spaces.

Working in millimetres and metres

Metric needs no conversion: add and subtract the millimetres. A 3600 mm wall less a 45 mm plate is 3600 − 45 = 3555 mm, or 3.555 m. In Metric mode each box takes mm, cm or m, so 2.4 m plus 600 mm is 3000 mm without changing units by hand. The answer also shows in feet and inches: 3555 mm ÷ 25.4 = 139.96", which is 11' 7 15/16" to the nearest 1/16". An inch is exactly 25.4 mm, so every conversion here uses that figure. More on reading metric as feet and inches in mm to feet and inches.

Area, volume and concrete

Area is length × width; volume is area × thickness. Work in metres for m² and m³, or in feet for ft², ft³ and cubic yards (27 ft³ to a yard).

• A 4200 × 3600 mm floor: 4.2 × 3.6 = 15.12 m², which is 162.75 ft² (1 ft² = 0.09290304 m²).
• A 4 × 3 m slab, 100 mm thick: 4 × 3 × 0.1 = 1.2 m³. Order 10% extra: 1.2 × 1.10 = 1.32 m³. In 20 kg premix bags at 108 a cubic metre, that's 1.32 × 108 = 142.6, so 143 bags.
• A 12 × 10 ft slab, 4 inches thick: 12 × 10 × 4/12 = 40 ft³, and 40 ÷ 27 = 1.48 yd³. With 10% extra, 44 ft³ or 1.63 yd³: 44 ÷ 0.60 = 73.3, so 74 bags of 80 lb.

The Concrete tab above does these sums. The concrete calculator adds post holes and strip footings, other bag brands, a cost from your own price, and rebar for a slab. How much extra to allow is covered in how much extra concrete to order.

Roof pitch, stairs and diagonals

These three are right-angle sums, and each tab shows its working answer at once.

• Roof pitch: a 6 in 12 roof rises 6 for every 12 across. Slope = 6 ÷ 12 = 0.5, so the grade is 50% and the angle is arctan(0.5) = 26.57°. Each unit of run takes √(1 + 0.5²) = 1.118 units of rafter. More in how to calculate roof pitch.
• Stairs: with a 240 mm going, the target riser is (600 − 240) ÷ 2 = 180 mm. A 2700 mm rise ÷ 180 = 15 risers of 180 mm and 14 treads; the run is 15 × 240 = 3600 mm and the stringer line √(2700² + 3600²) = 4500 mm. In feet and inches, a 9 ft rise with 10" treads gives 16 risers of 6 3/4" and 15 treads. The stair calculator checks the result against your country's limits.
• Diagonal: for a 3600 × 2400 mm frame, √(3600² + 2400²) = 4326.7 mm; for 12 × 8 ft, √(144² + 96²) = 173.07", which is 14' 5 1/16". When both diagonals of the frame measure that, it's square. See how to check a frame is square and the 3-4-5 rule.

Looking for a Construction Master calculator?

Construction Master Pro, from Calculated Industries, is a long-established feet-inch-fraction construction calculator, and many people search for it by name. Its Android app, Construction Master Pro Calc, lists feet-inch-fraction and metric maths with preset fractions from 1/2" to 1/64", rafter keys for regular and irregular hip, valley and jack rafters, stair layouts with a riser limit, and board feet. It starts with a free 14-day trial, then in-app purchases (Google Play listing, checked on 1 November 2026).

If you cut roofs or need those rafter keys, it does more than this page or MeisterCalc. If you need the everyday part, adding and splitting measurements, area, concrete, pitch, stairs and diagonals, this page does it free in a browser. For apps that cover similar jobs on Android, with their prices and what each lacks, see Construction Master Pro alternatives.

Questions people ask

What this calculator doesn't do

• No rafter, hip or valley cuts, birdsmouths or board feet.
• Stairs are straight flights from the total rise and going: no landings or winders.
• No length × length or length ÷ length in the dimension calculator; area and volume are in their tabs.
• It doesn't check building rules. Your local code, council or building department decides what complies.

The MeisterCalc app

Choosing an app? The best construction calculator apps for Android compares twelve, and is there a free construction calculator app? shows which MeisterCalc calculators are free and how that compares with other apps.

Related guides

• Feet and inches calculator
• Stair calculator
• Concrete calculator
• Roof pitch calculator
• A construction calculator app in millimetres