Cut a board square with a speed square
A crooked cut leaves gaps at a shelf end, a deck board or a piece of trim. The fix is a speed square, a small triangle with a lip that hooks on the board's edge. It gives you a true 90-degree line in seconds, and it can guide a circular saw too. First check that the board's factory end is square, because many aren't. Then measure, mark, and cut on the waste side of the line.
How a speed square gives you a square line
A speed square is a triangle of aluminum or plastic, usually 7 or 12 inches on a side. One edge has a thick lip, or fence, that hangs over the edge of the board. The straight side next to it is set at exactly 90 degrees to the lip. When the lip is pressed flat against a straight board edge, a line drawn along that side is square to the edge.
The same lip makes it a saw guide. This Old House calls it an instant fence: you press the lip to the board's edge, set the saw's base against the square, and run the saw along it. Pro Tool Reviews notes the method works for straight 90-degree and 45-degree cuts in boards up to about the square's size. A 7-inch square handles boards up to a 2x8, and a 12-inch square up to a 2x12.
Why a board cut comes out crooked or short
You measured from an end that was already off, so the new piece is too.
If the lip lifts off the edge, the line tilts with it.
A 1/8 inch misread or measuring the old, broken board.
The 1/8 inch blade kerf comes out of your piece instead of the waste.
An unclamped board or a sideways push pulls the cut off the line.
Binding can make the saw wander, burn the wood or kick back.
| You notice | Likely cause | Start at |
|---|---|---|
| Gap between the square and the board end | The factory end isn't square | Step 1 |
| New piece is too short or long | Measured from the wrong spot or misread the tape | Step 2 |
| Line looks crooked or wide | Lip not tight, or a dull pencil | Step 3 |
| Board shifts, sags or pinches the blade | Not clamped or supported right | Step 4 |
| Piece came out about 1/8 inch short | The kerf was on the wrong side of the line | Step 5 |
| Cut end has a slight gap or fuzzy edges | Trim and sand | Step 6 |
Let's fix it
- Check that the factory end is squareLumber ends from the store are often a little off, and some are split or crushed. Set the board flat on a table or sawhorses. Hook the lip of the speed square tight against the long edge, near the end. Slide it until its straight side touches the end of the board. If there's no gap, that end is square and you can measure from it. If you see a gap, or the end is split, plan to trim about 1/2 inch off first, using the same marking and cutting steps below. A clean, square starting end is what makes the finished piece square.
- Measure the length you needMeasure the space the board has to fit, not the old board, if the old one was broken or cut short. For a shelf, measure the back, middle and front of the opening, and use the shortest number. Subtract about 1/16 inch so it slides in without forcing. Hook the end of the tape on the square end of the board and pull it tight along one edge. At your number, draw a small V with the point exactly on the mark. A V shows the spot better than a dash. Measure again from the end before you cut.
- Hook the lip and draw the lineLay the speed square flat on the board with its thick lip, also called the fence, hooked over the near edge. Slide it along the edge until its straight side lines up with the point of your V. Press the lip firmly against the edge with one hand. If the lip lifts off the edge even a little, the line won't be square. With a sharp pencil, draw along the straight side from the near edge toward the far edge. A 7-inch square covers a board up to about 7 inches wide. For a wider board, draw as far as you can, then flip the square to the far edge and finish the line. Last, draw a big X on the side you'll throw away.
- Set up the saw and the board safelyPut on safety glasses, and hearing protection for a circular saw. Lay the board across sawhorses or a sturdy bench, with your line just past the support so the waste piece can fall away. Clamp the keep side down. Don't hold the board in your hand or across your knee. With the saw unplugged, set the blade depth so less than one full tooth shows below the board. That's the setting saw makers such as Bosch recommend, and it reduces kickback. Run the cord behind you, well clear of the cut path. Unplug the saw before changing or touching the blade.
- Cut on the waste side of the lineThe blade removes a strip of wood about 1/8 inch wide, called the kerf. Keep the whole kerf on the X side, so the pencil line stays on your piece. With a circular saw, set the speed square on the keep side with its lip hooked on the near edge. Rest the saw's flat base, the shoe, against the square's straight side and slide both until the blade lines up just beside your line. Hold the square firmly. Start the saw before the blade touches the wood, let it reach full speed, and push it steadily through. With a handsaw, start with a few light pull strokes at the far corner, then saw with long, easy strokes. Hold the waste end near the finish so it doesn't snap off.
- Check the cut, then sand the edgesHook the speed square on the long edge again and hold its straight side to the new end. There should be no light between them on either face. Then set the piece in place. A shelf should drop onto its supports with only light pressure. If the cut is slightly out of square or a hair long, mark a new line about 1/16 inch back and trim again. Sand off the fuzzy edges, called tear-out, with 120-grit sandpaper on a block. If your home was built before 1978 and you're cutting old painted trim, test the paint for lead first, and don't sand it if it's positive.
Watch it done
- The speed square shows no gap against the new end, on both faces
- The piece measures the length you marked, give or take 1/16 inch
- It fits its spot with light pressure, not a hammer
- The cut edges are smooth, with no splinters
If the cut still comes out wrong
If every cut comes out slightly angled even with the square tight, the saw blade may not be at 90 degrees to its base. Unplug the saw, set the bevel to zero, and hold the speed square between the base and the blade body, not the teeth. Adjust the bevel stop until there's no gap, as the saw's manual shows.
If the piece is square but a shelf still won't sit flat, the opening may not be square. Measure both diagonals of the opening; if they differ, cut to the shorter side and shim the gap. For trim, small gaps can be hidden with filler, as in our guide to filling nail holes in trim. A long splice in baseboard is a different joint, covered in replacing a damaged baseboard section.
If the board itself is warped, twisted or bowed, a square cut won't make it fit. Pick a straighter board, or cut a shorter piece from the flattest part. When you fasten the new board, our toenailing guide shows how to nail at an angle where you can't reach the end, and the hammer basics guide covers setting nails without dents.
What a pro typically charges (a rough estimate; prices vary by area): A handyman typically charges about $75–$150 for a small job like cutting and fitting a shelf or one board, often as a one-hour minimum. Lumber is extra.
Find a handyman near you