Online Set Squares / Triangle Rulers
Last updated: 2026-10-15
No set square handy? Check right angles, 45°, 30° and 60° right on your screen.
After calibrating the screen, the set square shows at true size.
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 1:1 physical set square.
What a set square is
A set square, also called a triangle ruler, is a flat drawing tool built around a true right angle. A pair usually comes together: one 45-45-90 triangle and one 30-60-90 triangle. The right-angle corner is used to square a line, and the other corners lay out fixed angles without measuring them.
This page is the on-screen version. Both triangles can be moved and rotated in the tools above, so you can compare them against a shape or read real lengths once the screen has been calibrated.
The two set squares
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.
How the side lengths relate
The 45° set square. The two legs are equal. If a leg is 8 in (about 20 cm), the hypotenuse is 8 × 1.414, roughly 11.3 in (28.7 cm).
The 30/60° set square. The shortest side faces the 30° corner. If the hypotenuse is 8 in (about 20 cm), the short leg is 4 in (10 cm) and the long leg is 4 × 1.732, roughly 6.9 in (17.6 cm).
These ratios let you check a drawing and confirm an angle without reaching for a protractor. Measure two sides, compare them with the ratio, and the corners are shown to be right.
Angles a set square can draw
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 set square 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 set square
Treat it like a physical set square: move it first, then rotate so a straight edge or corner lines up with your target.
Move
Drag the set square so its right-angle vertex or a straight edge sits where you need to measure.
Rotate
Use the Rotate button or the [ / ] keys to align the 45°, 30° or 60° edge with your target line.
Read or compare
Check the angle relationship; if you also need a length, finish calibrating the screen size first.
When do you need an on-screen set square
Reach for it whenever you briefly need to check a right angle, 45° or 30°/60°.
Rest a straight edge against the set square to see quickly whether lines are parallel or at right angles.
Click "Print" on a panel to print that set square at true 1:1 size. 10–15 cm fit A4; 20–30 cm need A3 or bigger paper.
Online set square in practice
Drawing a right angle. Put the 90° corner where the corner belongs, then draw along both legs. The corner stays square as long as the vertex has not been moved.
Drawing parallel lines. Rest one edge on the reference line, slide the triangle along without turning it, and draw along the same edge. The offset copies stay parallel.
Comparing an angle. Drag the triangle over a shape and line its 45°, 30° or 60° edge against the existing lines to see whether they match.
Working at true size. The on-screen set square shares the ruler's calibration. Check the screen against a bank card or the edge of a Letter sheet first, and once it is set the long edges read in real inches or millimetres.
Where a set square helps
Geometry constructions, dropping a perpendicular, bisecting an angle, and checking drawn corners.
Laying out perpendiculars, sloped lines and section marks on a plan.
Checking whether a wall corner is square, and marking 45° cut lines for a miter.
In sewing and craft, a set square trims pieces to a right angle before they are joined, which keeps a seam straight.
When you have no set square to hand
The on-screen version works anywhere a browser does, and after calibration it holds true size on the display. If you need it on paper instead, print the triangle at 100% so the millimetre graduations come out correct, and check a known length with a physical ruler afterwards.
When it matters that an angle is exact, cross-check it with the online protractor. Two tools agreeing on the same angle is a more reliable result than either one alone.
Frequently asked questions
Does the online set square 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 make sure the printed set square is true to 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.