OnCenter
Diag · common rafter0.022″ short

The feet-inch calculator for builders.

Punch in lengths just the way you'd read them off a tape, and OnCenter works the fractions out exactly, then shows you where the answer lands on a ruler. When the job calls for more, the builder tools take care of rafters, stairs, squaring and concrete, and you only ever pay for them once.

Coming soon to the App Store
The OnCenter calculator: a tape of board feet, rafter and stair answers above the yellow blade, which reads 13 feet 10 and 11 sixteenths inches with the red tick on the ruler, over the builder keys and keypad
01Rafter maths right on the keypad
The Rafter calculator's cut list for a 7/12 pitch over a 14 foot 4 inch span: line length, ridge off, shortened length, tail, and plumb and level cut angles, with the rafter total of 9 feet 4 and 9 sixteenths inches on the blade
02The whole rafter, with every cut
A fraction sum on the tape, 7 feet 3 and 5 eighths plus 2 feet 11 and 3 sixteenths equals 10 feet 2 and 13 sixteenths, and the blade showing 14 risers of 7 and a half inches
03Stairs worked out in one tap
The Takeoff calculator for a 10 by 10 foot concrete slab 4 inches deep: 1.23 cubic yards, or 56 eighty-pound bags
04How much concrete to order

What it does

  1. 01
    Feet, inches and fractions Free Type 7′ 3 5/8″ just as you'd read it off the tape, and choose how fine you want the answer, anywhere from a half inch down to a sixty-fourth. The blade always tells you how far the rounding moved it, so nothing slips past you.
  2. 02
    A tape that keeps your work Free Every answer drops onto a running tape, so you can tap a line to use it again or hold it to copy it somewhere else. And when a drawing comes through in metric, CONV switches the answer to inches, decimal feet or millimetres.
  3. 03
    Builder keys Pro Rise, run, diagonal, pitch and stairs each have their own key. Enter any two, and the next key you press solves the rest, right down to the plumb and level cuts.
  4. 04
    Rafters and squaring Pro Give it the span, pitch, overhang and ridge, and you'll get the rafter's total length along with a full cut list. Squaring up a slab or a deck? Enter the sides, measure the diagonal, and it'll tell you how far out you are and which way.
  5. 05
    Takeoff and board feet Pro Work out how much concrete to order, in cubic yards or 60 and 80 lb bags, along with drywall sheets, flooring with waste allowed for, and gravel or mulch. Board feet come with the cost once you add a price.
  6. 06
    Saved jobs and export Pro Give a tape a job name and it saves as you work, syncing through your own iCloud to your other devices. When the job's done, share it as text or a CSV, or export every job in one go.