Online Triangle Ruler
A triangle ruler — also called a set square — lets you check right angles and draw 45°, 30° and 60° lines right on your screen. Calibrate first if you need true-size lengths.
Place the right-angle vertex or a straight edge at the target, then rotate to the angle you need.
Print at 100% (turn off "fit to page") for a true-to-size physical triangle ruler.
How the two triangles work
A triangle ruler is a triangular drawing tool with a 90° corner and two acute angles. This page includes the two standard drafting triangles: 45°–45°–90° and 30°–60°–90°. Their straight edges help you check corners and lay out angles without measuring each one.
Move or rotate either triangle above to compare it with a drawing. Calibrate the screen only when you need to read real lengths.
How the triangles compare
Each triangle is defined by its three corners, and those corners also fix the ratio between its sides. That ratio is what makes the tool useful.
| Type | Angles | Side relationship |
|---|---|---|
| Isosceles right triangle | 45°, 45°, 90° | Two legs equal; hypotenuse = leg × 1.414 |
| Right triangle | 30°, 60°, 90° | Short leg = half the hypotenuse; long leg = short leg × 1.732 |
Between them the two triangles draw every whole multiple of 15°, from 15° up to 165°. That coverage is the reason they became the standard drawing pair.
Angles the pair can make
Used alone or slid against each other, the two triangles cover the angles below.
| Angle | How to get it |
|---|---|
| 15° | 45° − 30° |
| 30° | 30/60 triangle |
| 45° | 45 triangle |
| 60° | 30/60 triangle |
| 75° | 30° + 45° |
| 90° | Either triangle |
| 105° | 60° + 45° |
| 120° | 30° + 90° |
| 135° | 45° + 90° |
| 150° | 60° + 90° |
| 165° | 120° + 45° |
To combine them, rest one edge against the other and add or subtract the corners. Every angle you can produce is a multiple of 15°.
Common sizes
| Leg length | Typical use |
|---|---|
| 4 in (10 cm) | Student work, easy to carry |
| 6 in (15 cm) | General use |
| 8 in (20 cm) | Drafting, fits a Letter sheet |
| 10-12 in (25-30 cm) | Large drawings, needs bigger paper |
Students mostly carry the 4 in and 6 in sizes. Drafting and the trades favour 8 in and up, because a longer edge draws a longer straight line without stepping the tool along.
How to use the on-screen triangle ruler
Move the triangle into position, then rotate it until an edge or corner lines up with your target.
Move
Drag the triangle so its right-angle corner or a straight edge sits where you need it.
Rotate
Use the Rotate button or the [ / ] keys to align the 45°, 30° or 60° edge with your target line.
Read or compare
Compare the angles directly. If you need a length, calibrate against a known object such as a credit card first.
What you can do with it
Compare a drawn corner with 90°, or use a fixed edge for a quick angle check in geometry, craft or layout work.
Align a straight edge with a reference line to check whether another line is parallel or perpendicular.
Print either triangle at 100% for a physical copy. Sizes around 10–15 cm fit A4 or US Letter; 20–30 cm need larger paper.
Frequently asked questions
Does the online triangle ruler need calibration?
Only if you want true millimetre lengths. Angles and shapes are correct at any scale, but the 1 mm graduations correspond to real on-screen length only after you calibrate the screen.
Why doesn't the 90° angle need calibration?
An angle is a geometric relationship and has nothing to do with how long 1 mm is on screen. Screen calibration mainly affects the length scale.
Can I use it on a phone?
Yes, but the workspace is smaller. For jobs that need a large triangle or actual length measurement, a bigger screen is more comfortable.
How do I print a triangle ruler at true size?
Choose 100% or "Actual size" when printing and turn off "Fit to page / Shrink to page". After printing, it's best to check a known length with a physical ruler.
Related tools
Sources
The information on this page is based on published standards and reference works. For verification:
- Set squares and drafting tools: Wikipedia — Set square
- Triangles in geometry: Wikipedia — Triangle
- Technical drawing standards: Wikipedia — Technical drawing