Rafter Calculators

The length-and-cuts family is the heart of the site. The main Rafter Calculator takes three primary inputs — run, roof pitch, and overhang — and returns the full cut list a framer needs to lay out a common rafter: total rafter length, rafter line length, top length, plumb cut angle, seat cut angle, birdsmouth depth (HAP), heel cut, and setback. The tool supports imperial (ft/in with X/12 pitch) and metric (m with degrees) units, and exports a PDF cut list you can take to the saw.

The birdsmouth is the notch where the rafter seats on the wall plate. Getting it right is what makes a roof sit flat and plumb at the same time: the plumb cut follows the roof pitch, the seat cut is level, and the depth (HAP) has to leave enough wood above the notch to carry the load. The main calculator handles all three values live as you change pitch or run, and the advanced panel exposes rafter thickness, wall thickness, and ridge thickness for non-standard framing.

Span is the clear horizontal distance a framing member must bridge — the gap between bearing points that a joist or rafter has to cross without sagging. The span-and-sizing family answers one question: given a lumber size, a spacing, a species, and a live load, how far can this member span before it needs a bigger size, a tighter spacing, or an intermediate beam? The Floor Joist Span Calculator turns that question into a lookup against reference span charts for 2x6 through 2x12 lumber, with deck joist spacing and joist-count estimation built in.

Sizing is the inverse problem: given a target span, what size lumber do I need? The Rafter Span and Size Guide (a building-code reference guide, not a live calculator) walks through the IRC prescriptive span tables for rafters, including how to read live-load, snow-load, and spacing-by-species tables. For A-frame and shed-roof framing, where the rafter doubles as a wall stud or floor joist, the dedicated guides cover the geometry that the standard span tables do not address directly.

Roof pitch is the ratio of vertical rise to horizontal run, written as X/12 in North American framing (a 6/12 pitch rises 6 inches for every 12 inches of run). Pitch shows up on plans, in building-code span tables, on speed squares, and in manufacturer specifications — but each of those sources may express pitch differently: as X/12, as an angle in degrees, as a slope percentage, or as a rise-over-run fraction. The Roof Pitch Calculator converts between all of them in real time.

A few common conversions anchor the rest: a 4/12 pitch is about 18.4°, a 6/12 pitch is about 26.6°, and a 12/12 pitch is exactly 45°. The pitch triangle diagram above shows the geometry: the run is always 12, the rise is the pitch number, and the rafter is the hypotenuse. The angle is the arctangent of rise divided by run. For metric builders who think in degrees by default, the Metric Rafter Calculator guide covers the degrees-to-X/12 conversion and metric span tables.

The truss-and-framing family covers roofs that span the building with prefabricated trusses instead of site-cut rafters. The Roof Truss Calculator is an estimator and selection tool, not an engineering tool: enter your building span, truss spacing (24" residential or 48" agricultural), snow load region, and the truss type you are considering, and it returns a rough truss count, a span feasibility check against the selected family, and an auto-suggested alternative if your selection cannot cover the span.

The calculator compares six truss families side by side: King Post (16–26 ft spans), Queen Post (26–40 ft), Fink (28–60 ft, the residential default), Warren (30–60 ft, equal-triangle geometry), Scissor (vaulted ceilings), and Attic (room-in-attic for livable space). The Hip and Valley Rafter Guide covers the intersecting-roof geometry that trusses do not handle well — hip rafters, valley rafters, and jack rafters for L-shaped, T-shaped, and hip-roof plans.

The lumber-sizing and roofing-hub family covers two questions that sit just upstream of the cut list: what size lumber do I need? and which roofing calculation should I even run? The Rafter Lumber Size Calculator answers the first — it takes span, load, species, and on-center spacing, then returns the smallest nominal size (2x6 through 2x12) whose reference max span covers the input, plus its actual dressed dimensions and a margin check. It works as a 2x6 calculator, 2x8 calculator, 2x12 calculator, wood calculator, and beam span calculator in one tool.

The Roofing Calculator answers the second. It is a comparison-matrix hub that routes each roofing question to the right single-purpose track — pitch, rafter, span, or load — and explains the roof-load composition that feeds all of them. Where the calculators hub you are reading is the site-wide index, the Roofing Calculator is the roofing-specific entry point for roof calculator, roof framing calculator, and roof load calculator traffic. Use the lumber-sizing tool to pick a size before you cut; use the roofing hub to decide which calculation to run first.

Calculator Comparison

Side-by-side reference for every calculator on the site. Inputs and outputs are summarized so you can scan for the tool that matches the values you have and the values you need.

How to Choose the Right Calculator

The right calculator is the one that takes the values you already have and returns the values you need. Work backwards from your goal: if you need a cut list, you need the rafter calculator; if you need a span check, you need a span calculator; if you need a pitch in a different unit, you need the pitch calculator; if you need a truss count, you need the truss calculator.

1. I am cutting rafters for a gable, shed, or porch roof. Use the main Rafter Calculator. Enter run (half the building span, minus half the ridge thickness), pitch, and overhang. The tool returns length, plumb cut, seat cut, and birdsmouth depth in one pass, plus a PDF cut list. For shed roofs without a ridge board, set ridge thickness to zero.
2. I am laying out a floor system or deck. Use the Floor Joist Span Calculator. Enter your joist size (2x6 through 2x12), spacing (12", 16", or 24" o.c.), species, and live load. The tool returns the maximum allowable span and a joist count for the room dimensions. For decks, use the deck-joist spacing preset.
3. I have a roof pitch in X/12 and need it in degrees (or vice versa). Use the Roof Pitch Calculator. Enter any of: X/12 pitch, degrees, slope percent, or rise and run. The tool converts to all the others live. Use this for setting a saw bevel, a speed square, or a protractor.
4. I am estimating a truss package or comparing truss vs. rafter. Use the Roof Truss Calculator. Enter building span, truss spacing, snow load region, and the truss type you are considering. The tool returns a truss count, a span feasibility flag, and an auto-suggested alternative if your selection exceeds its typical span.

All Calculators

Quick-access grid of every calculator on the site. The main rafter calculator opens on the homepage at the calculator anchor; the three specialty calculators open on their own dedicated pages.

Related Carpentry Guides

Each calculator is backed by a written guide that explains the underlying geometry, the building-code references, and the manual layout method. The guides are organized under the guides index and linked from the relevant calculator pages; the full list is below for quick reference.

Frequently Asked Questions