Slat Count & Ladder Spacing Calculator
Enter a blind’s drop and slat width to estimate how many slats it holds, the ladder-tape pitch, and how tall the slats stack when raised.
Slats & ladder spacing
How the count is worked out
The calculator starts from the pitch— the slat width reduced by the overlap you choose — and divides the blind’s drop by it to find how many slats are needed to fill the window. Because the last slat has to reach the bottom, the count is rounded up. The rung spacing on the ladder tape equals that pitch.
The raised stack height multiplies the slat count by the slat thickness, giving a feel for how much of the glass a fully lifted blind will cover. It’s an estimate for planning and repairs — real blinds vary with the maker’s ladder tape, bottom rail, and headrail take-up.
Frequently Asked Questions
What is ladder spacing or pitch?
A venetian blind's slats hang from ladder tapes — pairs of cords with rungs between them. The spacing between rungs, called the pitch, is a little less than the slat is wide, so neighbouring slats overlap slightly and close tightly to block light. Pitch is what sets how many slats fill a given drop.
Why do the slats overlap?
If the rungs were spaced exactly one slat-width apart, closed slats would meet edge to edge and leak light through the gaps. Setting the pitch slightly shorter makes each slat overlap the one below when tilted shut, giving a solid, light-blocking face. This tool defaults to about a ten-percent overlap.
How does slat width change the count?
Wider slats mean a larger pitch, so fewer of them fill the same drop — a 50 mm blind uses roughly half as many slats as a 25 mm one over the same window. Narrow slats give a finer look and a shorter stack when raised; wide slats give a bolder look and a clearer view when open.
What is the raised stack height?
When you lift a venetian blind, the slats gather at the top. The stack height is roughly the number of slats times the thickness of each one, so thicker faux-wood slats stack taller than thin aluminium ones. It's useful for checking how much of the glass a raised blind will cover.
Estimates only. Ladder spacing, slat thickness, and stack take-up differ between manufacturers — measure your own blind before ordering replacement slats or tape.