Cat Bridge Calculator: Sag, Curve Length, Slat Gaps and Rope Pull

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Opens on the book's own cat and wall. Change any number to your own and press Calculate.

Span mm, step to lookout

Sag at the middle mm

The slats

Slats 0 = work it out from the gap

Or a target gap mm, optional; ¾ in = 19.05

Slat width mm; 1x4 = 88.9

Slat length mm; 8½ in = 215.9

Board length mm; 8 ft

Saw cut mm

The ropes and the cat

Ropes

Rope at each end for the knot mm

Rope

Cat's weight kg

Landing check × the weight

Enter the span and the sag and press Calculate.

The formula

Extra length of a shallow curve ≈ 8 × sag² ÷ (3 × span); along the curve = span + the extra
Equal gaps along the curve, a gap at both ends: (curve − n × slat width) ÷ (n + 1)
Pull in the ropes with a load P at the middle: H = P × span ÷ (4 × sag), shared by the ropes
Rope each side = curve + 2 × the tail for the knot

The book's worked example, Chapter 7. Across 1,168.4 mm with 120 mm of sag, the curve is 32.87 mm longer: 1,201.27 mm. 11 slats of 88.9 mm leave 12 gaps of 18.61 mm. A standing cat pulls the ropes with 143.28 N; a landing one with 859.65 N, 429.83 N in each, inside ½ in sisal's 630 N safe working load. Each rope is 1,801.27 mm with its tails, 3.6025 m for both; the slats take 2,407.9 mm of one 1x4.

The formulas are for a shallow curve, where the sag is small against the span, like the book's one-tenth. A bridge with almost no sag pulls its eye bolts very hard: never pull it tight. Metric first, imperial beside it. Prices are illustrative US home-center prices from November 2026, shown without a currency sign; put in your own. Everything stays in your browser: no sign-up, nothing stored.