Kerfy
A feet-inch-fraction calculator for people who build things. Type a measurement the way your tape reads it, and get an answer you can cut to.
For iPhone with iOS 17 or later.
A calculator that knows what a length is
- Exact, and says when it isn’t
- Kerfy does its arithmetic in exact fractions, not decimals that drift. 8′ divided by 7 shows as 1′ 1 11/16″ with a mark that says it was rounded to your tape’s precision, and one tap shows the exact value. Choose 1/8, 1/16, 1/32 or 1/64.
- Lengths, areas, volumes
- Add, subtract, multiply and divide feet, inches and fractions. Length times length gives square feet, and times a depth gives cubic feet and cubic yards. Mix in millimetres, centimetres and metres, converted exactly.
- A running tape
- Every calculation stays on the tape. Tap a line to use it again, or touch and hold it to see the exact value, copy it or save it to a job.
- Built for the jobsite
- Big keys you can hit with dusty or gloved fingers, and a high-contrast Sunlight theme for direct sun. It works with no signal, and there’s nothing to sign in to.
13 jobsite calculators
Every calculator opens with a worked example, so you can see what it does before you type. Save any result to a job and share it as text.
- Right triangleFree
- Rise, run, diagonal, pitch and angle. Check a layout for square.
- AreaFree
- Rectangles, triangles and circles, added up in square feet.
- Stairs
- Risers, riser height, treads, total run and stringer length.
- Roof pitch and rafters
- Rafter length with ridge and overhang, rise and angle.
- Roofing
- Roof area, squares and bundles.
- Concrete
- Slabs, footings and columns in cubic yards and bags.
- Stud and joist layout
- How many, and where every mark goes.
- Drywall
- Sheets for walls and ceiling.
- Paint
- Gallons and quarts for any number of coats.
- Flooring and tile
- Boxes or tiles, with waste.
- Fence
- Posts, sections, rails and pickets.
- Gravel and mulch
- Cubic yards, tons and bags.
- Board feet
- A list of boards that totals itself.
Numbers, not approvals
Kerfy gives you numbers. Stair and rafter results are geometry: what follows from the measurements you enter. Building codes vary by place, so check yours before you build.
Nothing leaves your iPhone
Kerfy has no account, no servers, and no advertising, analytics or tracking code. It doesn’t connect to the internet to do its work. Your tape, your saved jobs and your settings stay on your iPhone, and when you share a job, you choose where the text goes. The App Store privacy label reads “Data Not Collected”.
Free, or Kerfy Plus
Free
The calculator, the tape, unit conversions, the precision settings, and the right triangle and area calculators. No ads, and no time limit.
Kerfy Plus
Every calculator and saved jobs, for $14.99 once. No subscription, and Family Sharing covers your family.
US prices. Other App Store countries bill in their own currency.
Support
What’s free, and what does Kerfy Plus unlock?
The calculator, the tape, conversions and the precision settings are free, with the right triangle and area calculators. Kerfy Plus unlocks the other 11 calculators and the Jobs tab with one purchase ($14.99 in the US). A locked calculator still opens with its worked example solved, so you can see it before you buy; changing an input offers Kerfy Plus.
What does the ≈ mark mean?
Kerfy works in exact fractions. When an answer doesn’t land on a mark of your tape, the screen shows it rounded to the nearest one, with ≈ in front and a line saying so. Tap “Exact value” to see the exact figure and the same length in every unit.
How do I change the precision or the result format?
Tap the gear on the Calculator tab to open Settings. “Precision” sets the finest fraction a result is rounded to on screen, and “Show results as” picks feet and inches, inches, decimal feet or metric. The arithmetic stays exact either way.
Will Kerfy check my stairs or rafters against the building code?
No. Kerfy gives numbers, not approvals. Stair and rafter results are geometry, and building codes vary by place, so check the rules where you build.
How do I get Kerfy Plus back on a new iPhone?
Sign in with the same Apple Account, open Settings (the gear on the Calculator tab) and tap “Restore Purchases”. It’s on the Kerfy Plus screen too. Family Sharing covers the people in your family group.
How do I move my tape and jobs to a new iPhone?
Kerfy keeps them on your iPhone, with no account or cloud of its own. They’re part of your iPhone’s backup like any other app’s data, so setting up the new iPhone from your old one or from a backup brings them along. To keep a job anywhere else, share it as text.
How do I share a job?
Open the Jobs tab, then the job, and tap the share button. Kerfy writes every line as plain text, and you choose where it goes: Messages, Mail, Notes or anything else on the share sheet.
How do I delete my data?
Swipe left on a tape line or a job to delete it. Deleting the Kerfy app removes everything it stored on your iPhone. Nothing is kept anywhere else.
Does Kerfy need the internet?
No. The calculator and every calculator in it work with no signal. You only need a connection to buy or restore Kerfy Plus.
Something else? Write to us, and include your device, iOS version and what happened.
support@pupik.studio