How do I add and convert feet and inches for building work?

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Turn each length into inches, do the sum, and round only the answer to the nearest 1/16": 7' 8 5/8" + 4 1/2" is 92.625 + 4.5 = 97.125 in, or 8' 1 1/8". For a house, the IRC as your state or city adopts it caps stair risers at 7 3/4" and sets treads at 10" or more.

How do I add feet and inches without mistakes?

Convert both lengths to inches, add or subtract, then split the answer back into feet and inches. Rounding only at the end stops sixteenths from piling up.

Worked example: a stud cut at 7' 8 5/8" standing on plates that add 4 1/2".

  1. 7' 8 5/8" = 7 × 12 + 8.625 = 92.625 in.
  2. 92.625 + 4.5 = 97.125 in.
  3. 97.125 ÷ 12 = 8 whole feet (96 in), leaving 1.125 in: 8' 1 1/8" (2467.0 mm).

By parts it is the same: 8 5/8" + 4 1/2" = 13 1/8", which carries 12" into the feet. Subtracting works the same way, with a foot borrowed when the inches run short: 8' 1 1/8" − 6' 10 1/2" = 97.125 − 82.5 = 14.625 in, or 1' 2 5/8" (371.5 mm). An inch is exactly 25.4 mm, so 1/16" is 1.5875 mm, and a length rounded to the nearest sixteenth is out by 0.79 mm at most.

Free feet and inches calculator: in Add or subtract, enter 7 ft 8 5/8 and 0 ft 4 1/2 to check the sum above, or switch to Metric to feet and inches for a size off a metric drawing.

To multiply or divide a length, say a 13' 4" run split into 7 equal spaces, use the construction calculator, which works in feet, inches and fractions or in millimetres.

Does the MeisterCalc app work in feet and inches?

Yes, for input and answers, though it is metric first. Settings ▸ Units ▸ Default units ▸ Imperial (ft/in) turns every calculator over to feet, inches and fractions, with Fraction display and Fraction precision (1/8, 1/16 or 1/32) settings. It keeps every value in millimetres inside and converts for display. Its Feet & Inches calculator (Pro, new in version 1.1) adds or subtracts two lengths, such as 7' 8 5/8" and 4 1/2"; the web calculator above does the same in a browser.

Does my stair meet the IRC rise and run?

Under the 2021 IRC, a riser is 7 3/4" (197 mm) at most and a tread 10" (254 mm) at least (R311.7.5.1 and R311.7.5.2), and the tallest riser in a flight may not beat the shortest by more than 3/8" (9.5 mm). Headroom is 6' 8" (2032 mm), measured from the line of the nosings (R311.7.2). States and cities adopt the IRC with their own amendments, and some change these figures, so check your local code.

Worked example: 9' 1 1/4" floor to floor, finished floor to finished floor (109.25 in, 2775.0 mm).

  1. Fewest risers: 109.25 ÷ 7.75 = 14.1, so 15 risers of 109.25 ÷ 15 = 7.283", shown as 7 5/16" (185.0 mm), and 14 treads.
  2. Tread run at 10": 14 × 10 = 140 in, or 11' 8" (3556 mm).
  3. An easier stair: the comfort rule (twice the riser plus the tread is about 600 mm) gives a target riser of (600 − 254) ÷ 2 = 173 mm. 2775.0 ÷ 173 = 16.04, so 16 risers of 6 13/16" (173.4 mm), 15 treads and 12' 6" of run.
  4. Both pass: under 7 3/4", at least 10", and every riser the same.

The 3/8" rule is where site work goes wrong: plan the rise from finished floors, or flooring added later makes the top or bottom step differ from the rest. In the app, Stairs in Imperial with Rise Only, Total Rise 9' 1 1/4", Going 10" and Riser Limited ticked at a Desired Riser Height of 7 3/4" gives the 15 risers. The app doesn't do landings or winders. Laying out the stringer with a framing square is in how to lay out a stair stringer.

Free stair calculator: choose Feet and inches, set Check against to United States (IRC), and enter 9 ft 1 1/4 with a 10" going.

What stud spacing does the IRC allow?

16" or 24" on center, depending on the stud size, wall height and load, from Table R602.3(5) (section R602.3.1). For a 2×4 bearing wall up to 10 ft between supports, studs may be 24" on center when the wall carries only a roof and ceiling, and 16" when it also carries one floor. A 2×6 bearing wall carries one floor plus a roof at 24", or two floors plus a roof at 16". A 2×4 non-bearing wall up to 14 ft may be at 24". Utility grade studs are limited to 16".

Worked example: a 14 ft wall. Studs = length ÷ spacing, rounded up, plus one. At 16": 168 ÷ 16 = 10.5, up to 11, + 1 = 12 studs. At 24": 168 ÷ 24 = 7, + 1 = 8 studs. Both spacings divide a 4 × 8 sheet (48 ÷ 16 = 3, 48 ÷ 24 = 2), so sheet edges land on a stud. In metric, 16" is 406.4 mm and 24" is 609.6 mm, not 400 and 600. Corners, openings, headers and plates come on top; see how many studs do I need.

How far apart can balusters be?

Close enough that a 4" sphere can't pass (R312.1.3). The triangle under a stair's bottom rail must stop a 6" sphere (R312.1.3), and the guard on the open side of a stair must stop a 4 3/8". Guards are needed where the walking surface is more than 30" above the floor or grade below within 36" of the edge (R312.1.1), and they are at least 36" high, or 34" on stairs measured from the nosings (R312.1.2).

Worked example: 6 ft (72") between posts with 1 1/2" balusters. Baluster Spacing counts ⌈72 ÷ 4⌉ − 1 = 17 balusters with gaps of (72 − 17 × 1.5) ÷ 18 = 2.58", shown as 2 9/16". That keeps every gap under 4" but isn't the fewest: (72 − 4) ÷ (1.5 + 4) = 12.4, so 13 balusters with 3 3/4" gaps would also pass, worked by hand.

How much concrete in cubic yards and bags?

Cubic feet = length × width × thickness in feet; divide by 27 for cubic yards; add 10 percent. Bags are the order in cubic feet divided by the yield on the bag.

  • A 20 × 24 ft garage slab, 4" thick: 20 × 24 × 4 ÷ 12 = 160 ft³, or 5.93 yd³; 6.52 yd³ (176 ft³) to order, a ready-mix job.
  • Six deck footings 12" across and 42" deep (your plans and frost depth set the size): π × 0.5² × 3.5 = 2.75 ft³ each, 16.49 ft³ in all, 18.14 ft³ with 10 percent. QUIKRETE's data sheet gives 0.60 ft³ per 80 lb bag, 0.45 per 60 lb and 0.30 per 40 lb: 31 bags of 80 lb, 41 of 60 lb or 61 of 40 lb.

The app's Concrete takes feet and inches but shows the volume in m³ and litres (Total 0.4670 m³ for those footings), so use the web calculator for cubic yards and bag counts. Block walls are in the block calculator for CMU and brick.

Free concrete calculator: choose US (ft, in, yd³) and Post holes, then enter 12" across, 3 ft 6 in deep and 6 holes for the footing count above.

Which MeisterCalc calculators work for US jobs?

Stairs, Check Square and Concrete are free. Stud Counter, Running Measurements (layout marks at 16" or 24", read aloud with Speak), Baluster Spacing, Pitch (rise per 12 of run, degrees and percent), Raked Walls and Triangles are among the other 13, which unlock with a weekly subscription that starts with a 3-day free trial, or a one-time lifetime unlock; Google Play shows the prices in your currency. The app runs on Android 7.0 or later and works offline. It doesn't do feet-inch keypad arithmetic, board feet, rafter or hip cuts, or landings. Other trades and countries: calculators by trade and country.

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Codes and sources

Read on 1 November 2026. States and cities adopt the IRC in different editions and with their own amendments; your building department, the permit drawings and the makers' instructions decide what a job needs.

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