A-Frame Size Calculator

Use the Pythagorean theorem on half the cabin: rafter = sqrt((width/2)^2 + peak height^2). For a 20 ft wide A-frame with an 18 ft peak, that is sqrt(10^2 + 18^2) = 20.59 ft per rafter, with a base angle of 60.9 degrees (a 21.6-in-12 pitch). The Triangle mode above runs the full geometry — rafter length, base angle, pitch, cross-section area, footprint, roof surface, and perimeter.

Under IRC R305.1, rooms with sloped ceilings only count floor area where the ceiling is at least 5 ft high, and at least 50% of the counted area must have a ceiling of 7 ft or more. In an A-frame triangle, the width with a ceiling of at least y feet is width x (1 - y/peak height). Example: a 20 ft wide, 18 ft tall A-frame gives 14.44 ft of usable width (433.3 sq ft over a 30 ft length — 72.2% of the footprint), and the 7 ft zone passes the 50% check. The strips below 5 ft still work for storage.

IRC 2018 Appendix Q requires a sleeping loft to be at least 35 sq ft with every horizontal dimension at least 5 ft, and areas with less than 3 ft of headroom under the slope do not count. Example: in a 24 ft wide, 24 ft tall A-frame with the loft floor at 8 ft, the code-countable width is 13 ft — with a 12 ft deep loft that is 156 sq ft, comfortably above the 35 sq ft minimum. The Loft mode shows both the code-countable area and the area with 5 ft of comfortable headroom.

Plan the deck width as the cabin width plus 2 ft of overhang on each side, then pick depth by use: under 8 ft works as a walkway, 8-10 ft fits a bistro set, 10-12 ft a dining set, and 12 ft or more a lounge zone. Example: a 20 x 30 ft cabin with a 10 ft deep deck gives a 24 x 10 ft deck — 240 sq ft, or 40% of the cabin footprint. Standard deck sizes run 100-300 sq ft, with 300-400 sq ft counting as grand.

Put kitchens and baths against an end wall, where the full triangle height is available. A room of width W needing H feet of ceiling fits when W is less than cabin width x (1 - H/peak height). Example: in a 24 ft wide, 24 ft tall A-frame, an 8 ft wide room needing a 7 ft ceiling fits easily — the triangle allows up to 17 ft at that height. The Kitchen/Bath mode checks your exact room and tells you the minimum cabin width if it does not fit.

Plan 150 sq ft per person for cozy or 200 sq ft per person for roomy — so 4 people means a 600-800 sq ft target, and remember an A-frame loses the strips below the 5 ft ceiling line. The calculator recommends a 28 x 36 ft footprint with a 26 ft peak for 4 people, which yields 814.2 sq ft of usable floor after the 5 ft rule. A sleeping loft on top adds beds without enlarging the footprint.

Per ASCE 7, the sloped-roof snow load is Ps = Cs x Pf, where Pf = 0.7 x Ce x Ct x Is x ground snow load. Steep A-frame slopes shed snow: at 63.4 degrees the warm-roof slope factor Cs drops to 0.16, so a 40 psf ground load becomes just 4.6 psf on the roof. Example: a 24 ft wide, 24 ft tall A-frame (32 ft long) at 40 psf ground snow needs 2x12 rafters at 16 in on center (SPF No.1/No.2). At 70 psf with 24 in spacing, even 2x12 is not enough — the tool flags it for an engineer.

Common stock plans cluster around 20-24 ft wide and 30-36 ft long: 20x36, 24x30, and 24x32 ft are the most typical footprints, with peak heights of 18-26 ft and steep 21 to 24-in-12 roof pitches (about 60-63 degrees). Width is usually fixed by the plan while length is flexible — you can shorten or elongate most designs. Start from the Triangle mode with your width and peak height to see the exact geometry.

This calculator gives planning-level sizing — geometry, usable area under code ceiling rules, loft and deck proportions, and a simplified ASCE 7 snow-load rafter check (bending plus L/240 deflection, SPF No.1/No.2, 875 psi). It is not a substitute for stamped structural drawings: connections, foundations, wind and seismic loads, and local code amendments all need a licensed engineer in your jurisdiction. Use the Snow mode to sanity-check rafter sizes before you pay for full design.