Lining a tennis court means transferring the ITF’s official rectangle onto the ground from a single center reference line, then proving every corner is square with a diagonal measurement instead of a framing square. A doubles court’s full diagonal measures 85 ft 11 in (26.18 m); a singles-only court’s diagonal measures 82 ft 6 in (25.15 m). If a taped or painted corner does not match one of those two numbers, the rectangle is out of square before a single cross line goes down.
Start from the official dimensions, not a tape-measure guess
Every length and width below comes from Rule 1 of the 2026 ITF Rules of Tennis, already sourced on this site’s tennis court dimensions guide. The two diagonal rows are not part of Rule 1. They are calculated from those same official dimensions with the Pythagorean theorem and labeled as derived calculations, not rules.
| Measurement | Figure | Applies to | Status |
|---|---|---|---|
| Court length | 78 ft (23.77 m) | Singles and doubles | Official rule |
| Doubles width | 36 ft (10.97 m) | Doubles | Official rule |
| Singles width | 27 ft (8.23 m) | Singles | Official rule |
| Net to each service line | 21 ft (6.40 m) | Singles and doubles | Official rule |
| Doubles alley width, each side | 4 ft 6 in (1.37 m) | Doubles | Derived calculation |
| Doubles court diagonal (squaring check) | 85 ft 11 in (26.18 m) | Doubles | Derived calculation |
| Singles court diagonal (squaring check) | 82 ft 6 in (25.15 m) | Singles | Derived calculation |
Set the four corners before marking a single line
A rectangle is easiest to lay out from its corners, not by chasing one line at a time. Stretch a string line long enough to run past both ends of one baseline, and mark the center point of that line. From the center point, measure out half the doubles width, 18 ft (5.48 m), to each side to fix the two baseline corners. From each of those corners, run a second string the full 78 ft (23.77 m) court length toward the net, and mark the opposite corner at that distance. Repeat from the other baseline corner so all four corners come from the same two reference measurements: half-width and length.
Only after all four corners are fixed should the sidelines, net line, and inner singles lines go down, using the net-to-baseline and net-to-service-line figures from the table above. Marking corners first and connecting them second keeps a small error in one measurement from compounding into every line that follows.
Check every corner with the diagonal, not a framing square
Once the four corners are marked, measure both full diagonals of the court, corner to opposite corner. On a doubles court, each diagonal should read 85 ft 11 in (26.18 m); on a singles-only court, 82 ft 6 in (25.15 m), as in the table above. If the two diagonals do not match each other, the rectangle is not square, even when each side measures the correct length on its own. A parallelogram can have four correct side lengths and still be crooked; only equal diagonals confirm the right angles.
A smaller check works at any single corner and does not require the full court to be staked out first: measure 3 ft along one line from the corner and 4 ft along the line that should meet it at a right angle, then measure the distance between those two points. On a true right angle, that third side is exactly 5 ft, the classic 3-4-5 right triangle. Doubling those numbers to 6, 8, and 10 ft gives a longer baseline and a more precise check. This is a general property of right triangles, not a tennis-specific rule, but it catches a square corner early, before string lines are moved or paint goes down.
Line widths and where the surveyed point sits
Rule 1 sets most line widths between 1 and 2 in (2.5-5 cm), with the baseline allowed up to 4 in (10 cm) and the center service line fixed at exactly 2 in (5 cm). The ITF measures every court dimension to the outside edge of the relevant line, not its centerline or its middle. That detail matters once tape or paint has width: the corner point established by the string lines and diagonal check is the outer edge of the baseline and the outer edge of the sideline, not the center of the painted stripe. Laying the line’s outer edge on that surveyed point, rather than centering the line over it, keeps the playing surface at the full official size instead of shrinking it by the width of the paint.
Overhead plan
The same surface seen from straight above, drawn to scale. Nothing is foreshortened here, so every figure can be read off the drawing the way a tape measure would take it.
Detail sheet
A rule book fixes more than the shape of the surface, and the rest of it is too small to draw at court scale: a mark a few centimeters long, a post standing a meter off the sideline, the clearance around the whole thing. Each one here has its own frame and its own scale.
Frequently asked questions
Which line marks the back of a tennis court?
The baseline marks each end of the court. The two baselines sit 78 ft (23.77 m) apart and are shared by singles and doubles play.
Do singles and doubles need separate sets of lines?
No. A single court is marked once. Doubles adds alleys 4 ft 6 in (1.37 m) wide outside the singles sidelines, and both formats share the same baselines, service lines, and center service line.
How do I know if a marked court is actually square?
Measure both full diagonals. They should match each other and equal 85 ft 11 in (26.18 m) for doubles or 82 ft 6 in (25.15 m) for singles. Matching side lengths alone do not prove square corners.
Can pickleball lines be added to an existing tennis court?
Yes, in a contrasting color, since a tennis court’s surface is large enough to hold pickleball lines inside it. The exact overlay geometry and how many pickleball courts fit are covered in the pickleball-on-tennis-court guide.
How wide should tennis court lines be painted?
Rule 1 allows 1 to 2 in (2.5-5 cm) for most lines, up to 4 in (10 cm) for the baseline, and exactly 2 in (5 cm) for the center service line. Any width inside those ranges is legal.
Official sources
- International Tennis Federation, 2026 Rules of Tennis, Rule 1. Verified July 26, 2026, as cited on this site’s tennis court dimensions guide.
Diagonal measurements checked against these dimensions on October 8, 2026. The methodology page explains how each figure is labeled as a rule or a derived calculation; report an error through corrections.