Construction calculators that show their work
Construction calculators that cite their constants, handle imperial and metric properly, and flag the cases where the arithmetic is right but the answer fails inspection.
Most used: square footage, feet and inches, area units, price per square foot, rebar, stairs, roof pitch, tile. Every calculator ends with a note on what a practising architect checks before trusting the number on a job; the About page says how each page is tested before it ships. The calculators moved here from tools.formgiving.ai on 14 October 2026; every old address redirects to the same page on this host.
Concrete & cement
6 calculators
- Cubic Yard CalculatorVolume of slabs, holes, footings and cylinders in cubic yards, feet and metres, several pieces added, with the depth in inches.
- Cement Bags Calculator — 50 kg bags, sand and aggregate in CFT50 kg cement bags, sand and aggregate in CFT for a nominal mix (1:2:4, 1:1.5:3) from a volume or slab size; 1.54 and 1,440 kg/m³ marked as convention.
- Concrete Volume CalculatorConcrete volume for a slab, footing or column in cubic yards and cubic metres, with a realistic waste factor and bag counts from published yields.
- Asphalt Tonnage CalculatorHot-mix asphalt by the ton from area and compacted thickness, with the unit weight editable because the real one comes from the mix design.
- Plaster Calculator — cement bags and sand in CFTCement bags and sand in CFT for cement-sand plaster: wet and dry volume at a stated factor, split by ratio. Which figures are exact, which are convention.
- Stucco Calculator — bags by the coat, and plaster by the wallBags of base coat and finish coat from the wall area and the coat thickness at the data sheet's coverage, and cement and sand for site-mixed plaster.
Masonry & block
5 calculators
- Concrete Block (CMU) CalculatorBlock count, courses, mortar and grout for a concrete block wall, with the exact blocks-per-square-foot arithmetic shown.
- Brick Calculator — bricks, mortar and the bed jointBrick count from the nominal face module, mortar by the cubic foot, and the bed joint coursing really needs: 5/12 in on modular, not the 3/8 in on the spec.
- Block Calculator — 9 inch and 6 inch sandcrete blocks, mortar and cementBlocks for a wall in the 450 × 225 mm sandcrete block (9 in, 6 in): exact count from face plus joint, openings deducted, mortar and 50 kg bags as convention.
- Hollow Block Calculator — CHB per square metre, mortar and cell fillingCHB for a wall from the 400 × 200 mm module: blocks per m², courses, laying mortar in 40 kg bags, cell filling and a bar count from your spacing. Philippines.
- Retaining Wall Calculator — blocks and baseBlock count, buried base course and drainage stone for a retaining wall, with the height at which engineering and permits enter the picture.
Rebar & steel
2 calculators
- Rebar Calculator — bars, laps and spacing checkBar count and weight for a slab, including the lap splices most calculators omit and a check against the ACI 318 maximum spacing.
- TMT Bar Weight — kg per metre, per piece, per tonneMass per metre for 6 to 32 mm TMT bars, then pieces, bundles and tonnes. Where d²/162 comes from, and the 0.121% it runs heavy by.
Roofing
4 calculators
- Roofing Shingle Calculator — squares, bundles and pitch areaRoofing squares and shingle bundles from a building footprint, with the pitch multiplier that makes a roof larger than the house beneath it.
- Roof Pitch ConverterConvert roof pitch between rise-per-12, degrees, percent and rafter multiplier, and get rafter length for any run.
- Metal Roofing Calculator — panels, squares and screwsPanels by count and length for a gable roof, from the footprint, the pitch and the panel's cover width, with squares, screws at a stated rate and the trim runs.
- Attic Ventilation CalculatorNet free vent area from the IRC R806.2 ratio, split between eave intake and ridge exhaust, turned into a ridge length and a soffit vent count.
Framing & lumber
4 calculators
- Wall Framing Calculator — studs and platesStud count, plate footage and framing factor, with a direct comparison of 16 and 24 inch on-centre spacing.
- Board Feet Calculator — nominal vs actualBoard feet, linear feet and cost for framing lumber, showing both the nominal size you are billed for and the actual surfaced size you build with.
- Plywood and Sheathing CalculatorSheets of plywood or OSB for a floor, roof or wall from area and sheet size, with waste, rows and the offcut, and fasteners from your schedule.
- Deck Board CalculatorDeck boards, joists and fasteners, with the installation gap that differs between wet treated lumber and pre-dried or composite boards.
Insulation
2 calculators
- Assembly R-Value Calculator — nominal vs effectiveSee what a framed wall actually delivers once the studs bridge the insulation, using the parallel path method with a framing factor.
- Insulation Calculator — batts, rolls and coverageBatts, rolls and blown-in by bag count. Bag coverage falls as the target R rises, so the same square footage takes 1.57 bags of R-21 for every bag of R-13.
Flooring
4 calculators
- Tile and Flooring CalculatorTiles, boxes and attic stock for a room, with a waste allowance sized to the lay pattern.
- Laminate Flooring CalculatorBoxes from the room and the carton's coverage, rows and the last-row width, with the waste the lay pattern costs and the expansion gap the count must leave.
- Vinyl Plank Flooring CalculatorBoxes of LVP from the carton's coverage, rows, last-row width, and whether the product wants an underlayment roll or forbids one.
- Carpet CalculatorSquare yards from the room and the roll width, counted in strips because the roll is, with stairs, waste and an optional installed price.
Walls & finishes
7 calculators
- Paint Coverage CalculatorWall area minus openings, times the number of coats, rounded up to whole cans — and how much is left for touch-ups.
- Drywall CalculatorSheets, screws, tape and joint compound for a room, plus a comparison of board lengths and the joints each one leaves you.
- Wallpaper CalculatorRolls from the wall, the roll on the label and the pattern repeat, counted in strips because a roll yields whole strips and nothing else.
- Siding CalculatorWall area less openings, then lap siding by the piece from its exposure or panel siding by the sheet, with gables counted as triangles.
- Drop Ceiling Tile and Grid CalculatorTiles, main tees, cross tees, wall angle and hanger wires for a suspended ceiling, with the hanger spacing ASTM C636 sets.
- Crown Molding Angle CalculatorMiter and bevel for a compound cut from the corner and spring angles — the two formulas behind every chart — plus linear feet and sticks.
- Board and Batten Calculator — boards, battens and linear feetBoards and battens from the wall length and the lumber's dressed face — exterior with a gap, or interior at a spacing — with linear feet and board feet.
Stairs & slope
3 calculators
- Stair Rise and Run Calculator — IBC and IRC checkRiser height, tread depth, total run and stringer length from a total rise, checked against IBC 1011.5 and IRC R311.7 limits, which are not the same.
- Slope and Grade CalculatorConvert slope between percent, ratio, degrees and fall over a run, and see the fall for any run length.
- Drain Pipe Slope CalculatorThe fall a horizontal drain needs over its run from the IPC 704.1 minimum for the pipe size, and your own slope in inches per foot, percent and ratio.
Earthwork & aggregate
5 calculators
- Excavation Calculator — swell, haul and backfillBank, loose and compacted volumes for an excavation: how much you dig, how much you haul, and why the hole will not fill itself.
- Gravel and Aggregate CalculatorAggregate volume and tonnage for a driveway or base course, with a compaction allowance and the density caveat that decides whether a delivery arrives short.
- Sand Calculator — cubic yards, tons and bagsSand for a base, bedding or a sandbox: volume from area and depth, loose volume with compaction, tons at a stated density, and bags.
- Mulch and Topsoil CalculatorCubic yards and bags of mulch, topsoil or compost for a bed, plus guidance on the depth that actually helps.
- Paver Calculator — pavers, bedding sand and basePavers, bedding sand and compacted base for a patio, including the base that runs past the edge restraint and makes the excavation larger than the patio.
Fencing & outdoor
4 calculators
- Fence Calculator — posts, rails and picketsPosts, rails, pickets and fasteners for a wood fence, cut from the dressed sizes PS 20 sets and honest about which of the other numbers are only convention.
- Fence Post Concrete CalculatorConcrete per post hole and total bags for a fence, with embedment depth checked against the one-third rule and a frost depth warning.
- Sod Calculator — what a pallet really coversSod by piece and pallet. A pallet sold as 450 sq ft carries 440 sq ft of pieces, so the order is sized from piece count, not the advertised figure.
- Pool Volume Calculator — gallons and litresGallons from the pool's shape and its shallow and deep ends, with the exact 231 in³ gallon, in gallons, litres and cubic metres.
Land area units
15 calculators
- Square Feet to Pyeong and Area Unit Converter — sq ft, m², pyeong, tsuboConvert square metres, square feet, Korean pyeong, Japanese tsubo, acres and hectares through exact legal definitions rather than rounded factors.
- Katha to Square Feet: 720, 1,361, 2,880 or 3,645, by RegionKatha (kattha, cottah) to square feet, square metres and gaj by region: Kolkata 720, Guwahati 2,880, Patna about 1,361, Nepal 3,645. Each figure graded.
- Bigha Converter — by state, and where there is no answerHectares and acres into bigha, katha, biswa and dhur, by the measure that governs the land. Rajasthan fixes six bighas by rule; seven regions get no number.
- Marla and Kanal Converter — by districtMarla, kanal, sarsahi and ghumao against the karam length in force. Punjab recognises four karams, so the same 5 marla plot spans a 49% range by district.
- Cent, Ground and Guntha ConverterCent, guntha and ground against the exact acre, keeping the two that are real fractions of an acre apart from the one that is only a Chennai convention.
- Gaj to Square Feet — the plot unit, not the cloth unitOne gaj of land is one square yard: 9 ft² and 0.83612736 m², exact. The plot sizes people quote in gaj, and the 33-inch length gaj that is not this unit.
- Decimal to Square Feet: 435.6 Exactly, and What the Roundings CostOne decimal (dismil, satak) is 435.6 ft², 48.4 gaj and 40.4686 m², 1/100 acre exactly. Same area as a cent; 0.605 Kolkata katha. What 400 and 436 do to a plot.
- Dhur to Square FeetOne dhur is 182.25 ft², 16.93 m² and 20.25 gaj in the Nepal Terai, about 68 ft² in Patna; 20 to the katha, 400 to the bigha. Each figure graded, four refusals.
- Plot to Square Metres — 60 × 120, 50 × 100, and why there is no "the plot"A 60 × 120 ft plot is 668.9 m²; 18 × 36 m is 648. Enter length × width or pick a convention: m², ft², acres, and exact plots per acre against the quoted six.
- Chatak to Square Feet: 45 ft², a Sixteenth of the Kolkata KathaChatak (chhatak) to square feet, m² and gaj: 45 ft², 16 to a 720 ft² Bengal katha, 320 to the bigha. Reads and writes "3 katha 5 chatak"; refuses the weight.
- Ankanam to Square FeetOne ankanam is 72 ft², 8 gaj and 6.689 m² by the Nellore convention, 6.05 to the cent, 605 to the acre. No document fixes it; Tirupati's 36 is refused.
- Biswa to Square Feet: 453.75, 871.2 or 1,361.25, by MeasureBiswa to square feet, square metres and gaj by measure: Delhi 453.75, Rajasthan 132 ft chain 871.2, pucca and 165 ft chain 1,361.25. Biswansi too. Each graded.
- Perch to Square Feet and Square Metres: the Sri Lankan Land Perch, Exactly1 perch = 272.25 ft², 30.25 yd², 25.2929 m² exactly, 160 to the acre. Turn a deed in acres, roods and perches into ft² and m² and back; price per perch.
- Ground to Square Feet — the Chennai plot unit, gradedOne ground is 2,400 ft², 222.97 m², 266.67 sq yd and 5.51 cents by Chennai convention; 18.15 to the acre. Where the figure comes from, and the wrong numbers.
- Guntha to Square FeetOne guntha is 1,089 ft², 121 gaj and 101.17 m² exactly, 40 to the acre. What the 1,000 ft² and 100 m² roundings lose, and why an are on a 7/12 is not a guntha.
Area & cost
3 calculators
- Square Footage CalculatorArea of a room, a lot or an odd shape from rectangles, triangles and circles added together, in square feet, yards, metres and acres with exact factors.
- Lot Size Calculator — acres, square feet and metresA lot's area from frontage and depth in square feet, acres, square metres and hectares with exact factors, and any acreage drawn as a square.
- Price per Square Foot CalculatorPrice divided by area both ways, in square feet and square metres, with two listings side by side and a rate-times-area check.
Drawing & dimensions
2 calculators
- Architectural Scale ConverterDrawing scale to real size at any imperial, engineering or metric scale, answered in feet-inches to the nearest 1/16 inch with the rounding residual shown.
- Feet and Inches Calculator — add and subtractAdd and subtract feet, inches and sixteenths as a running dimension string, with a quantity on every row and the residual left by rounding to 1/16.
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