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How to use the construction calculator
- Keypad. Type a length the way you say it: 5 Feet 3 Inch 1 / 2 is 5' 3-1/2". Put +, −, × or ÷ between lengths and press =. Every entry and total lands on the tape beside the keys.
- Triangle and pitch. Type any two of rise, run, diagonal and pitch. The other two fill in, with the plumb and level cut angles.
- Stairs. Enter the total rise from finished floor to finished floor. The tool picks the number of risers closest to your target riser height, sizes the treads and checks each rule of the code you choose.
- Roof framing. Enter the span, pitch and overhang. You get the common rafter, the hip or valley, a jack rafter cut list and the roof area in squares.
| You press or type | Shown as | Notes |
|---|---|---|
| 5 Feet 3 Inch 1 / 2 | 5' 3-1/2" | Feet before inches, inches before the fraction |
| 5 / | 5/16" | No bottom number: the fraction setting is used |
| 130 Feet Feet | 130 sq ft | Press a unit twice for area, three times for volume |
| 3.2 m, 450 mm | 3.2 m, 450 mm | Metric works on the keypad and in every form |
| Conv, then a unit key | 14.625 ft | Conv Feet twice switches between 14' 7-1/2" and 14.625 ft |
8' 10-1/4", 106.25, 2700 mm | form fields | A bare number in a stair field is read as inches (millimetres in metric mode) |
Like a job-site calculator, the keypad works left to right with no precedence, so a + b × 2 means (a + b) × 2. That is how you total two walls and double them in one pass.
Reading feet-inch-fraction answers
The tool keeps every number in full precision and rounds only what it shows, to the nearest fraction you pick, with a half rounding up, the way you read a tape. The line under the display gives the exact decimal and how far it was rounded. Squaring a 15' 6" by 10' 2" foundation gives a diagonal of √(186² + 122²) = 222.441 inches. At 1/16 that reads 18' 6-7/16", rounded down by 0.0035 inch. At 1/4 it reads 18' 6-1/2".
| Fraction setting | 1/2 | 1/4 | 1/8 | 1/16 | 1/32 | 1/64 |
|---|---|---|---|---|---|---|
| 7.4 inches reads as | 7-1/2" | 7-1/2" | 7-3/8" | 7-3/8" | 7-13/32" | 7-13/32" |
Units follow the maths. A length times a length is an area in square feet, a third length makes a volume in cubic yards (tap cu ft under the display for cubic feet), and a length divided by a length is a plain count: 25' ÷ 3' 6" = 7.142857, so you get 7 whole pieces. One inch is exactly 25.4 mm, so 10' 6" is 3.2004 m.
Rafter length per foot of run, hips and jacks
Pitch is the inches of rise for every 12 inches of run, so 6/12 climbs 6 inches per foot. The run of a common rafter is half the span. By Pythagoras its line length per foot of run is √(12² + pitch²) inches, so the line length is run × √(144 + pitch²) ÷ 12. A hip (or valley) sits at 45° in plan, so it travels 16.97 inches for every 12 inches of common run and its length per foot of common run is √(288 + pitch²). That is why a 6/12 hip is called "6 in 17".
| Pitch | Angle | Common rafter, in per ft of run | Hip or valley, in per ft of run |
|---|---|---|---|
| 4/12 | 18.43° | 12.65 | 17.44 |
| 6/12 | 26.57° | 13.42 | 18.00 |
| 8/12 | 33.69° | 14.42 | 18.76 |
| 9/12 | 36.87° | 15.00 | 19.21 |
| 12/12 | 45.00° | 16.97 | 20.78 |
These are the same numbers stamped on a framing square's rafter table. Jack rafters run from the wall to the hip, and each one is shorter than the last by the spacing × the common rafter's length per inch of run, the "common difference". Line lengths run to the centre of the ridge or hip, so the tool also gives the deductions: a common rafter loses half the ridge thickness measured level, a hip with a single cheek cut loses half the 45° thickness of the ridge (1-1/16" level for a 1-1/2" ridge), and jacks lose half the 45° thickness of the hip and take a 45° cheek cut. At the wall, the birdsmouth heel is the seat cut × pitch ÷ 12.
How the stair rules work
The number of risers is the total rise ÷ your target riser, rounded to a whole number. Each riser is the total rise ÷ that count, there is one tread fewer than risers because the upper floor is the last step, and the stringer runs from the first nosing to the last: treads × √(riser² + tread²). The tool then checks the result:
| Rule | IRC R311.7 (US) | Approved Document K, private stair (England) |
|---|---|---|
| Riser height | 7-3/4 in (196 mm) maximum | 150 to 220 mm |
| Tread depth (going) | 10 in (254 mm) minimum | 220 to 300 mm |
| Even steps | largest within 3/8 in of the smallest | rise and going consistent |
| Headroom | 6' 8" (2032 mm) | 2.0 m (1.9 m for loft conversions) |
| Other | 151 in maximum rise per flight; 3/4 to 1-1/4 in nosing when treads are under 11 in | pitch 42° maximum; 2R + G from 550 to 700 mm |
Headroom over a stair depends on the floor opening above it. Measured up from the line of the nosings, it is the opening length × riser ÷ tread, less the thickness of the upper floor at the opening's edge. States and cities amend the IRC, and Scotland, Canada and Australia set their own values, so treat these checks as a first pass, not an approval.
Four worked examples
1. Baseboard and floor area for a room
A room measures 13' 7-3/8" by 11' 2-5/8". With chain logic, 13' 7-3/8" + 11' 2-5/8" = 24' 10", and × 2 gives 49' 8" of baseboard before you take out the doorways. Multiply the two walls instead and you get 152.7386 sq ft of floor.
2. Concrete for a slab
A slab 24' by 18' 6" and 4" thick is 24 × 18.5 × 4/12 = 148 cu ft, which the keypad shows as 5.481481 cu yd. Add a 10% allowance with + 10 % and order 6.02963 cu yd, about 6 yards.
3. Basement stairs with a 9' 1-1/2" rise
109.5 ÷ 7.5 = 14.6, so the tool picks 15 risers of 7.3 inches, marked as 7-5/16". With 10" treads there are 14 treads and the total run is 11' 8". Each step's diagonal is √(7.3125² + 10²) = 12.388 inches, so the stringer measures 14 × 12.388 = 14' 5-7/16" from the first to the last nosing, at 36.18°. Stepping off 7-5/16" fifteen times ends 3/16" high, inside the 3/8" limit, but the cumulative marks table avoids it. With a 10' 6" opening and a 10-1/2" floor, headroom is 126 × 7.3 ÷ 10 − 10.5 = 81.48 inches, or 6' 9-1/2", which passes 6' 8".
4. A 6/12 hip roof, 28' by 40'
The run is 14'. The common rafter's line length is 14 × 13.416 = 187.83 inches, or 15' 7-13/16". Less half a 1-1/2" ridge it is 15' 7", and a 16" overhang makes the cut length 17' 0-7/8". The hip is 14 × 18 = 252 inches, exactly 21' 0", at 6 in 17 (19.47°). At 16" spacing each jack is 17-7/8" shorter than the last, from 14' 1-15/16" down to 8-15/16", with 8 of each length. The roof covers 1,462.9 sq ft, or 14.63 squares, and the ridge is 12' long.
Common mistakes
- Measuring the rise to the subfloor. Total rise is finished floor to finished floor. Add the flooring you have not laid yet.
- Using the span as the run. A gable or hip rafter covers half the span.
- Cutting to the line length. Line length is to the ridge centre. Take off the ridge deduction and add the tail.
- Mixing up pitch and degrees. 6 in 12 is 26.57°, not 6°. Pick the right unit next to the pitch box.
- Stepping off rounded numbers. Small rounding adds up over 15 steps. Mark from the running totals instead.
Open a problem from a link
Copy link saves the open tab with its inputs, so a link opens the exact problem:
| Parameter | Example |
|---|---|
tab | keypad, triangle, stairs or roof |
calc | ?calc=24 ft x 18 ft 6 in x 4 in (operators need spaces around them) |
prec, units | prec=32, units=met |
| Triangle | rise, run, diag, pitch, pu=in12|deg|pct |
| Stairs | rise, target, tread, run, risers, opening, floor, code=irc|adk|none |
| Roof | type=hip|gable, span, pitch, overhang, length, ridge, oc |
Embed the Construction Calculator on your website
Teachers, bloggers and course sites can add this tool to any web page for free. Paste the code below into an HTML or embed block (WordPress, Google Sites, Canvas, Moodle, Blogger, Wix). It loads the same tool you see above, without our menus or ads. Please keep the credit line under the frame. Sizes, a live preview and step-by-step guides for each platform are on the calculator widget page.
Frequently asked questions
Is this a Construction Master calculator?
It is a Construction Master style calculator: an independent design that follows the same key sequences, such as 5 Feet 3 Inch 1 / 2 for 5' 3-1/2" and Conv then a unit key to convert. WideMath is not affiliated with Calculated Industries, which makes the Construction Master, and this page runs free in your browser.
How many stairs do I need for a 9 foot rise?
With a 7-1/2 inch target, 108 ÷ 7.5 = 14.4, so you need 14 risers of 7.714 inches (7-11/16" on the tape) and 13 treads. With 10 inch treads the total run is 10' 10", and the riser is inside the IRC maximum of 7-3/4 inches.
What is the rafter length for a 6/12 pitch?
A 6/12 common rafter is 13.42 inches long for every foot of run. On a 24 foot span the run is 12 feet, so the line length is 12 × 13.416 = 161 inches, or 13' 5", before the ridge deduction and the overhang.
What are the IRC rules for stair risers and treads?
IRC R311.7 allows a riser of at most 7-3/4 inches and a tread of at least 10 inches, with the largest riser or tread no more than 3/8 inch over the smallest. Headroom must be 6' 8", and one flight can rise at most 151 inches between floors or landings.
How do I convert decimal feet to feet and inches?
Keep the whole feet and multiply the decimal part by 12. For 14.625 feet, 0.625 × 12 = 7.5, so it is 14' 7-1/2". On the keypad, type 14.625 Feet, then press Conv and Feet.
Can I use metric units?
Yes. The keypad has m, cm and mm keys, and the triangle, stair and roof tabs have a metric switch that shows millimetres. The stair tab can check against England's Approved Document K, and one inch is exactly 25.4 mm throughout.