How We Build and Test Our Calculators

This page explains how every BuildByCalc calculator is built, where its numbers come from, and how we test it.

1. Every constant has a source

A calculator is only as good as its constants: how much a block covers, how many blocks a bag of mortar lays, how much a cubic yard of gravel weighs. We take these from, in order of preference:

Industry estimating guides and standards bodies (for example NCMA/CMHA for concrete masonry).
Manufacturer data sheets for the bagged products people actually buy (for example Quikrete).
Supplier conversion sheets, only where the first two don’t cover a material.

Where sources disagree, we use the figure that won’t leave you short, and say so on the page. For example, NCMA says an 80 lb bag of mortar mix lays about 16 blocks and Quikrete says 13, so we use 13.

2. Rounding and waste

Things you buy whole are rounded up: blocks, bags and pieces. You can’t buy 141.75 blocks.
Waste is shown, never hidden. Where a waste allowance applies, it is a field you can change, and the result shows the count before and after waste.
Units: lengths can be entered in feet, inches, yards, meters or centimeters. Volumes are shown in cubic feet and cubic yards.

3. Testing

Each calculator has a set of hand-worked test cases: the worked example on its page, a metric case, and edge cases such as zero or very large inputs. The arithmetic for each case is written out by hand first, and the calculator has to reproduce every number before it is published. The worked example on each page is printed straight from those tested results.

4. Prices

Material prices vary a lot by region and change often, so every cost field can be edited. Default prices are examples to show how the estimate works. Enter your local prices for a real budget.

5. Calculator notes

Concrete block calculator

Quantity	Method	Source
Blocks	Net wall area × 1.125 (an 8 × 16 in face covers 8/9 sq ft), plus your waste allowance	NCMA TEK 4-2A
Mortar mix (80 lb)	Blocks before waste ÷ 13	Quikrete Mortar Mix data sheet
Masonry cement (70 lb)	Blocks ÷ 30; sand at 1 ton per 8 bags, plus ½ ton above 3 tons	NCMA TEK 4-2A
Core-fill grout	Cu ft per 100 sq ft by wall width and grout spacing (3% waste included); an 80 lb bag yields 0.66 cu ft	NCMA TEK 4-2A, Tables 3–4

Concrete calculator

Quantity	Method	Source
Slab or footing	Length × width × thickness (× how many), plus your extra allowance	Geometry
Post hole or column	π × (diameter ÷ 2)² × depth × how many. The post itself isn’t subtracted, so the result is slightly generous	Geometry
Solid steps	Width × run × rise × (1 + 2 + … + number of steps)	Geometry
Bags	Cubic feet ÷ 0.60 (80 lb), 0.45 (60 lb) or 0.30 (40 lb), rounded up	Quikrete Concrete Mix data sheet

Gravel calculator

Quantity	Method	Source
Volume	Area × depth, plus your extra allowance; ÷ 27 for cubic yards	Geometry
Tons	Dry gravel 105 lb/ft³ (1.42 t/yd³); wet 125 lb/ft³ (1.69 t/yd³); or your supplier’s figure	Engineering ToolBox
Bags	Cubic feet ÷ 0.5 (50 lb bag)	Quikrete All-Purpose Gravel

Asphalt calculator

Quantity	Method	Source
Tons	Square yards × compacted inches × 110 lb ÷ 2,000, plus your extra allowance. The rate is editable	Tennessee DOT (uses 105 lb/yd²/in); 110 adds a small margin

Topsoil calculator

Quantity	Method	Source
Volume	Area × depth, plus your settling allowance	Geometry
Tons	Loam 80 lb/ft³ (1.08 t/yd³), or your supplier’s figure	Engineering ToolBox
Bags	Cubic feet ÷ bag size (0.75, 1 or 1.5 cu ft), rounded up	Retail bag sizes

Cubic yard calculator

Quantity	Method	Source
Volume	Area × depth; 27 cu ft per cubic yard; 0.0283168 m³ per cu ft	NIST Handbook 44
Optional weight	Gravel dry 105, topsoil 80, sand dry 97, sand wet 119 lb/ft³, or your supplier’s figure	Engineering ToolBox

Board foot calculator

Quantity	Method	Source
Board feet	Thickness (in) × width (in) × length (ft) ÷ 12; one board foot is 144 cubic inches	Virginia Department of Forestry

Mulch calculator

Quantity	Method	Source
Volume	Area × depth; one cubic yard covers 324 sq ft at 1 inch	Geometry
Bags	Cubic feet ÷ bag size (1.5, 2 or 3 cu ft), rounded up	Retail bag sizes
Depth guidance	2–4 inches; a warning above 4 inches	University of Illinois and University of Vermont Extension

Sand calculator

Quantity	Method	Source
Tons	Dry sand 97 lb/ft³ (1.31 t/yd³); wet 119 lb/ft³ (1.61 t/yd³); or your supplier’s figure	Engineering ToolBox
Bags	Cubic feet ÷ 0.5 (50 lb bag)	Quikrete All-Purpose Sand

Sod calculator

Quantity	Method	Source
Sod to order	Lawn area plus your extra allowance	Geometry
Pieces	Square feet ÷ piece size (16 × 24 in slab = 2.67 sq ft; 2 × 5 ft roll = 10 sq ft), rounded up	Sod Solutions; Angi
Pallets	Square feet ÷ your pallet size (typically 450–500 sq ft), rounded up	Sod Solutions

Landscape rock calculator

Quantity	Method	Source
Tons	Area × depth ÷ 27 × 1.42 t/yd³ (dry gravel, 105 lb/ft³), or your supplier’s figure	Engineering ToolBox
Bags	Cubic feet ÷ 0.5, rounded up	Vigoro bagged landscape rock

Paver calculator

Quantity	Method	Source
Pavers	Area plus your extra allowance ÷ one paver’s area, rounded up	Geometry
Base gravel	Minimum compacted depth 4 in (patios, walkways) or 6 in (home driveways), plus your compaction allowance; 105 lb/ft³	ICPI Construction Tolerance Guide; Engineering ToolBox
Bedding sand	1 in layer; dry sand 97 lb/ft³; 0.5 cu ft bags	ICPI; Engineering ToolBox

Retaining wall calculator

Quantity	Method	Source
Buried depth	At least 6 in, or 1 in per foot of wall height if more	Allan Block
Blocks	Courses (height + buried ÷ block height, rounded up) × blocks per course, plus your extra	Geometry
Base gravel	Trench 18 in wide (24 in for walls 4 ft and over) × 6 in deep, plus your compaction allowance	Allan Block
Drainage gravel	12 in behind the blocks for the full height including the buried course	Allan Block

Fence calculator

Quantity	Method	Source
Posts	Length ÷ spacing, rounded up, + 1, + 1 per corner and per gate	Geometry
Holes	Diameter 3 × post width; depth one third of the post’s overall length + 6 in gravel	Quikrete Setting Posts guide
Concrete	(Hole area − post area) × (buried depth − 3.5 in); 0.60 cu ft per 80 lb bag, 0.375 per 50 lb	Quikrete
Pickets	Board run ÷ (picket width + gap), plus your extra	Geometry

Deck calculator

Quantity	Method	Source
Rows and boards	Deck width ÷ (board width + gap), rounded up; × boards per row; plus your extra	Geometry
Joists	Deck length ÷ spacing (12, 16 or 24 in), rounded up, + 1	Trex
Fasteners	2 screws or 1 hidden clip per board per joist, plus 5%	Board makers’ installation guides

Concrete cost calculator

Quantity	Method	Source
Ready-mix total	Yards × your price per yard + delivery + short-load fee ((minimum − yards) × fee, when under the minimum) + pump	Your quotes; defaults are labelled examples
Bagged total	Yards × 27 ÷ 0.60 bags (80 lb), rounded up, × your bag price	Quikrete Concrete Mix data sheet
Break-even	The number of yards where both totals are equal	Algebra

Square footage calculator

Quantity	Method	Source
Area	Rectangle L × W; L-shape as two rectangles; circle π r²; triangle base × height ÷ 2; × number of areas	Geometry
Conversions	÷ 9 for sq yd; × 0.09290304 for m²; ÷ 43,560 for acres	NIST Handbook 44

Stair calculator

Quantity	Method	Source
Risers and treads	Total rise ÷ target riser, rounded up; treads = risers − 1	Geometry
Code checks	Riser ≤ 7¾ in, tread ≥ 10 in, handrail at 4+ risers, 6 ft 8 in headroom	IRC R311.7 (as quoted by Kitsap County)
Stringer	√(rise² + run²), rounded up to the next even board length	Geometry

Roof pitch calculator

Quantity	Method	Source
Pitch, angle, slope	Rise ÷ run × 12; arctan(rise ÷ run); rise ÷ run × 100	Trigonometry
Roof area	Footprint × √(1 + (rise ÷ run)²); squares = area ÷ 100	Trigonometry

Drywall calculator

Quantity	Method	Source
Sheets	Walls (perimeter × height − openings) and ceiling, each plus your extra ÷ sheet area, rounded up	Geometry
Screws	About 28 per 4 × 8 sheet on walls and 35 on ceilings, scaled by sheet area (screws 16 in o.c. on walls, 12 in on ceilings)	USG
Tape and compound	370 ft of tape and about 9 gal (two 4.5 gal pails) of all-purpose compound per 1,000 sq ft	USG

Flooring and tile calculators

Quantity	Method	Source
Boxes	Area plus your extra ÷ the square feet per box printed on the carton, rounded up	Your product
Tiles	Area plus your extra ÷ ((tile length + joint) × (tile width + joint)), rounded up	Geometry
Baseboard	Perimeter − 3 ft per doorway	Geometry

Paint calculator

Quantity	Method	Source
Gallons	(Walls − 21 sq ft per door − 15 sq ft per window, plus ceiling if chosen) × coats ÷ coverage (default 350 sq ft per gallon)	Behr: 250–400 sq ft per gallon

Rebar calculator

Quantity	Method	Source
Bars each way	(Slab dimension − 2 × edge distance) ÷ spacing, rounded down, + 1	Geometry
Sticks	Runs that fit a stick share sticks; longer runs are lapped (your lap in bar diameters) and use whole sticks	Geometry
Weight	#3 0.376, #4 0.668, #5 1.043, #6 1.502 lb per ft	ASTM A615, Table 1

Roofing calculator

Quantity	Method	Source
Roof area	Footprint × √(1 + (pitch ÷ 12)²), plus your extra; squares = ÷ 100	Trigonometry
Bundles	Area ÷ 32.8 sq ft (3 bundles cover 98.4 sq ft), rounded up	GAF Timberline HDZ
Nails	64 shingles per square × 4 nails (or 6)	GAF
Drip edge	Eaves and rakes ÷ 10 ft pieces, rounded up	Geometry

Insulation calculator

Quantity	Method	Source
Existing R-value	Depth × 2.5 (loose fiberglass), 2.8 (rock wool), 3.7 (cellulose) or 3.2 (fiberglass batts)	DOE Building America, Table 2.2
Target	R30 (zone 1), R49 (zones 2–3), R60 (zones 4–8), or your own	ENERGY STAR (2021 IECC)
Inches to add	(Target − existing) ÷ the new material’s R per inch	DOE Building America

6. Limits

Our calculators estimate materials. They are not structural designs. Walls that retain soil, walls over 4 ft, footings and anything load-bearing may need an engineer and a permit. Check with your local building department.

Spotted something wrong? Let us know. We correct errors quickly and update the date on the page.