Rafter Length Calculator

Common & Hip Rafter Length, Angles, Birdsmouth and Count

The Rafter Length Calculator gives you the numbers you actually cut to: the common rafter body length from the ridge to the wall plate, the overhang tail, the total board length, the run and rise, the plumb and seat cut angles, the maximum birdsmouth depth, and the diagonal hip rafter length for four-sided roofs. Enter the building span, the roof pitch, the overhang, the ridge board thickness and your rafter depth, and the tool applies the pitch multiplier — the square root of one plus rise over twelve squared — to the run to get the sloped length, then deducts half the ridge thickness. Rafter span is horizontal, not sloped, which is the single most common mistake in DIY framing. Add the roof length and spacing and you also get the rafter count and total lineal feet of lumber to order. Pair it with the Rafter Span Chart to confirm your lumber size carries the span, and with the Roof Truss Calculator if trusses are the better route.