Siding & Exterior: Calculators, Diagrams & Guides
3 calculators · 4 diagrams · 2 guides
A siding takeoff has two ways to go wrong: the wall area (forgetting gables, subtracting too much or too little for openings) and the coverage math (lap boards cover their exposure, not their width — a 8¼-inch plank at 7-inch exposure covers 15 percent less wall than its face suggests). The calculators in this hub handle both, producing plank or panel counts for fiber cement, LP SmartSide, vinyl, and metal, plus housewrap rolls, trim boards, flashing, fasteners, and sealant.
The diagrams show the assembly the way a spec sheet should: exposure and overlap geometry, coursing that lands evenly at windows, butt-joint flashing, clearances at grade and roofline, and the full weather-resistive barrier stack behind the cladding. Fiber cement and engineered wood both live or die on these details — most warranty claims trace to a clearance or flashing miss, not the product.
Manufacturer installation specs are the governing documents here — James Hardie’s requirements for HardiePlank, LP’s for SmartSide — alongside IRC Chapter 7 for water-resistive barriers. The hub’s three siding calculators are scoped so you can price a whole exterior or a single trade: the general siding hub tool for panels and comparisons, and dedicated lap-siding and HardiePlank tools tuned to those products. Free, no signup.
Siding & Exterior calculators
- Siding CalculatorInstantly calculate siding panels, trim & accessories for vinyl, fiber cement, wood or metal. Doors and windows subtracted automatically. Free, no signup.
- Lap Siding CalculatorFree generic lap siding calculator for LP SmartSide, wood bevel, clapboard & vinyl. Boards per square, exposure, trim, corners & fasteners. Even coursing.
- HardiePlank CalculatorFree HardiePlank calculator — boards, HardieTrim, kickout & butt-joint flashing, fasteners & sealant by HardieZone (HZ5/HZ10). Hardie HS11119 & IRC.
Guides & references
Siding & Exterior · 4 diagrams
- Siding & Exterior
Lap siding exposure vs. board width vs. overlap — why an 8¼-inch board covers only 7 inches
A lap board hides part of itself: the overlap is covered by the course above, so an 8¼″ board shows only 7″. Exposure (width − overlap) is what drives how much siding you buy — not the board width.
- Siding & Exterior
Even coursing for lap siding — avoiding the sliver top row under the soffit
Running every board at max exposure leaves an ugly sliver under the soffit. Even coursing shrinks the reveal just enough (108″ ÷ 16 = 6¾″) so all courses are equal and the top one lands clean at the eave.
- Siding & Exterior
Gross vs. net wall area for siding — subtracting windows and doors, then adding waste
Size a siding order by net area, not the whole wall: subtract the windows and doors from the gross wall, then add 10–15% for cuts and coursing.
- Siding & Exterior
Lap siding wall assembly — sheathing, house wrap, starter strip, flashing, and ground clearance
What goes on behind lap siding, bottom to top: sheathing, lapped house wrap, a starter strip that sets the first course angle, the nailed courses, and base flashing — with the siding held at least 6″ above finish grade.
Frequently Asked Questions
How do I calculate how much siding I need?
Measure each wall (width × height), add gable triangles at half base × height, subtract openings, then divide by the coverage of one plank at its EXPOSURE — face width minus the overlap. The siding calculator automates the geometry and applies a 10 percent waste factor, or 15 percent on complex layouts with many corners and openings.
What is the difference between plank width and exposure?
Exposure is the part of the board that shows after the course above overlaps it — typically the plank width minus 1¼ inches. An 8¼-inch HardiePlank at the standard 7-inch exposure covers only 7 inches of wall height per course, so ordering by plank width under-buys by roughly 15 percent, the single most common lap-siding mistake.
What clearances does fiber cement siding require?
The big ones: 6 inches to grade, 2 inches above roofs and decks, 1 inch above flashing at horizontal trim, and a ⅛-inch caulked gap at trim per the Hardie spec (butt joints in field areas are flashed, not caulked). Violating clearances is the fastest route to a voided warranty and wicking damage at board bottoms.
Do I need housewrap behind siding?
Yes — the IRC requires a water-resistive barrier behind exterior cladding in nearly all assemblies. Standard rolls run 9 or 10 feet by 100 to 150 feet; overlap 6 inches horizontally and 12 at verticals, taped at seams. Every siding calculator in this hub includes the wrap and tape takeoff with the plank count.