Remember what the hypotenuse is. That’s the long side of a right triangle — and it’s why the 3-4-5 trick has been squaring corners and figuring duct offsets since before any of us picked up snips. Three out, four up, and if the diagonal hits five, you’re square. Every time.
The calculator below does the math for you. Type your rise, get your run and cheek length, down to the sixteenth. Works on your phone, on the ladder, on the floor, no app to download.
Figure a Duct Offset (3-4-5 Calc)
Enter your rise — the distance the duct has to move over. Get the run and the cheek length.
Cheek length is point to point. Add your flange allowance for cleat connections on each end before you cut.
Square a Corner
Any multiple of 3-4-5 gives the same true 90°. Enter a multiplier, measure the three sides, and if the diagonal checks, your corner is square.
Feet or inches — the ratio doesn’t care, as long as all three sides use the same unit.
What to Do With the Numbers
The calculator gives you three numbers. Here’s how I put them on the floor.
Find a straight line to work off. I use a framed wall base track or edge against the wall — that’s your baseline. Come off the wall the distance of your rise and make a mark. Then pull your hypotenuse from the starting point on the baseline to that mark. When the tape hits the number, you’re square — same 3-4-5 you just ran.
Now set the duct. Put an assembled piece against the wall and a free piece at the end of the hypotenuse. Run the tape over everything one more time to double-check — it’s a lot cheaper to find a mistake now than after you cut. When it checks, mark the corners on the floor.
Set a piece of duct on the marks, mark the duct off the floor, and draw your lines solid before you cut. Then cut, notch for the cleat, trim the insulation, and bend your drives. Do the same on the other piece, and assemble.
Both ends get an s-cleat. Even when a duct terminates into an end cap or ties into another branch, it still needs a flange — either to screw to the next piece if it’s terminating into a duct, or an s-cleat and drive to hold the end cap. That’s why the calculator adds two inches by default. The one-cleat option is there for the odd piece where you only lap one end.
Pro tip: draw the lines solid, every time. A tick mark looks fine until you’re halfway through a cut and can’t remember which side of it you meant. Account for s-cleat lappage. If you added it in the calculator, you will be too long.