Find your frame's outside size. The diagonal is what both corner to corner measurements should read when it's square. The second table is the 3-4-5 rule, for squaring one corner before the frame is built.
Deck, shed and slab frames
| Frame | Diagonal |
|---|---|
| 8 × 8 ft | 11' 3 3/4" |
| 8 × 10 ft | 12' 9 11/16" |
| 8 × 12 ft | 14' 5 1/16" |
| 10 × 10 ft | 14' 1 11/16" |
| 10 × 12 ft | 15' 7 7/16" |
| 10 × 16 ft | 18' 10 7/16" |
| 12 × 12 ft | 16' 11 5/8" |
| 12 × 16 ft | 20' |
| 12 × 20 ft | 23' 3 7/8" |
| 14 × 20 ft | 24' 4 15/16" |
| 16 × 16 ft | 22' 7 9/16" |
| 16 × 20 ft | 25' 7 3/8" |
| 20 × 20 ft | 28' 3 7/16" |
The 3-4-5 rule
| Mark along each side | Diagonal |
|---|---|
| 3 ft and 4 ft | 5' |
| 6 ft and 8 ft | 10' |
| 9 ft and 12 ft | 15' |
| 12 ft and 16 ft | 20' |
| 15 ft and 20 ft | 25' |
How this is worked out
A rectangle is square when its two diagonals are equal. The diagonal is the square root of one side squared plus the other side squared. We work it out in inches and round to the nearest sixteenth.
If one diagonal is long, the two corners it runs between are open and need to come in. Push them toward each other and measure both again. Handyman Calc calls a frame square when a measured diagonal is within 1/4 inch of the number here. That's the usual carpentry figure. Cabinets want tighter.
The 3-4-5 rule works because 3, 4 and 5 make an exact right triangle. Mark 3 ft along one side and 4 ft along the other. When the marks are 5 ft apart, the corner is square. On a bigger frame use 6, 8 and 10 or 9, 12 and 15. The longer the sides, the smaller any error gets.
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Measured a diagonal and it's off? Type it into Handyman Calc and it says how far out you are and which way to push. Try Handyman Calc free in your browser. There's nothing to install and no account.
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The chart is on one page in the printable PDF, US Letter size.
More charts: how many bags of concrete a slab takes, how many sheets of drywall a room takes, a fraction to decimal chart for inches, how much paint a room takes, how much mulch a bed takes, a roof pitch chart, how many steps a rise takes and board feet for common lumber.
To work out your own sizes, there's a material estimator spreadsheet, and more on the Handyman Calc page.