← AskMachinist · Guides · Calculator · 🔎 Search
How to Use a Combination Square
Guide · Updated October 27, 2026
The combination square is a rule with interchangeable heads. It does more layout jobs than any other single tool.
Need an exact reference size?
Build any gage block stack with the free Gage Block Calculator. Free, no login.
Step by step
- Square and miter: Set the square head on the edge for 90° and 45° lines.
- Depth and height: Slide the rule to the depth, lock it, and use it as a depth gage or to set a height.
- Center head: Set it on round stock and scribe two lines at an angle. They cross at the center.
- Protractor head: Set it to the angle and scribe or check.
- Check it: Scribe a line, flip the square and scribe again. Any gap between the lines is twice the error.
Worked example: checking the square head
Draw a line along the blade with the head on a straight edge of a board or plate. Flip the head over and draw a second line beside the first from the same point. If the lines diverge by 0.010 in over 6 in, the square is out by half that: 0.005 in over 6 in, about 0.0008 in per inch. Check against a machinist square or gage blocks on a surface plate for work tighter than that.
Using the center head
Put the center head on round stock and scribe along the blade; turn about 90° and scribe again. The crossing is the center. On a 2.000 in bar a 1° error in rotation does not matter; what matters is that both V faces touch.
Using the protractor head
Set the protractor to the angle, lock it, and lay out or check the bevel. Most protractor heads read in whole degrees, so use a sine bar for anything tighter.
More common problems
Blade slips: lock nut worn. Readings off at the 45°: dirt in the groove. Rule wear at the end: measure from the 1 in line.
Tips
- Keep the rule clean and the lock snug.
- Don't use it as a hammer or pry bar.
- For precision squareness work, see checking squareness.
Related: layout dye, checking squareness.
FAQ
How do I check if a combination square is square?
Scribe a line, flip the square and scribe again. Any spread between the lines is twice the error.
How accurate is a combination square?
Good ones are around 0.001 in per 6 in. Use a master square for precision work.
How does the center head work?
It places the rule's edge through the center of round stock. Two lines cross at the center.
Can it set heights precisely?
Roughly. For precision, set heights from gage blocks. Build any reference size free with the Gage Block Calculator at gageblockcalc.com.
Related guides and calculators
- How to build a gage block stack
- How to wring gage blocks
- How to choose a gage block set
- Gage block care and wear blocks
- Carbide vs steel vs ceramic gage blocks
- How to check a height gage with gage blocks
- Machinist Christmas gift guide
- How to use a bevel protractor
- How to Check a Machinist Square
- How to Use a Center Finder
- How to Use a Hermaphrodite Caliper
- Sheet Metal Bend Allowance on a Box and Pan Brake
- All free machinist calculators
Build your stack in seconds
The free Gage Block Calculator finds the fewest-block combination for inch and metric sets, with wear blocks. No login.
Questions? Ask a machinist.
General shop information only — follow your gage manufacturer's instructions and your quality system.