Methodology

How Project Math calculators work

Every calculator on this site uses a published formula, editable defaults, and the same rounding rules. This page lists all of them so you can check the math yourself.

Last reviewed .

Principles

  • Standard geometry. Quantities come from area and volume formulas: length times width for area, and area times depth for volume.
  • Label-based constants. Values such as paint coverage, concrete bag yield, and bundles per roofing square reflect typical product labels. Where products vary, the value is an input you can change.
  • Waste is explicit. Each calculator adds a visible waste percentage instead of hiding a buffer in the formula.
  • Round up to what you buy. Results are rounded up to whole gallons, boxes, sheets, bags, boards, or bundles, because stores do not sell fractions.
  • Planning only. The tools estimate quantities. They do not design structures, replace code requirements, or account for site conditions.

Paint Calculator

Formula

  1. Wall area = 2 x (length + width) x ceiling height
  2. Paintable area = wall area - (doors x 20 sq ft) - (windows x 15 sq ft)
  3. Coverage needed = paintable area x coats x (1 + waste %)
  4. Gallons = coverage needed / coverage per gallon, rounded up

Assumptions and defaults

  • Each door removes 20 square feet and each window removes 15 square feet.
  • Coverage defaults to 350 square feet per gallon, the low end of most interior paint labels.
  • A 10 percent buffer is added before rounding up to whole gallons.
  • Walls only. Ceilings, trim, and primer are separate purchases.

Mulch, Soil, and Gravel Calculator

Formula

  1. Cubic feet = length x width x (depth in inches / 12)
  2. Adjusted cubic feet = cubic feet x (1 + waste %)
  3. Cubic yards = adjusted cubic feet / 27
  4. Bags = adjusted cubic feet / bag size, rounded up

Assumptions and defaults

  • The area is a rectangle. Split odd shapes into rectangles and add the results.
  • An 8 percent buffer covers uneven ground and settling.
  • Bag size defaults to 2 cubic feet, the most common mulch bag.
  • Volume only. Gravel and soil are often sold by weight, so ask the supplier for tons per cubic yard.

Flooring Box Calculator

Formula

  1. Floor area = length x width + second area length x second area width
  2. Adjusted area = floor area x (1 + waste %)
  3. Boxes = adjusted area / box coverage, rounded up
  4. Material cost = boxes x price per box

Assumptions and defaults

  • The room is a rectangle plus an optional second rectangle. Use the second area for the other leg of an L-shaped room, a closet, or a hallway, and leave it at 0 otherwise.
  • Waste defaults to 10 percent, which suits straight plank layouts.
  • Box coverage defaults to 23.5 square feet. Always replace it with the number on your product.
  • Underlayment, transitions, and trim are not included in the cost.

Tile Calculator

Formula

  1. Area = length x width
  2. Adjusted area = area x (1 + waste %)
  3. Tile area = (tile length x tile width in inches) / 144
  4. Tiles = adjusted area / tile area, rounded up
  5. Boxes = adjusted area / box coverage, rounded up

Assumptions and defaults

  • Waste defaults to 15 percent to allow for edge cuts in small rooms.
  • Grout joints are ignored, which keeps the tile count slightly on the safe side.
  • Box coverage defaults to 15 square feet. Replace it with the number on your box.
  • Thinset, grout, trim, and waterproofing are listed as reminders but not sized.

Drywall Sheet Calculator

Formula

  1. Wall area = room perimeter x wall height
  2. Total area = wall area + ceiling area - openings
  3. Adjusted area = total area x (1 + waste %)
  4. Sheets = adjusted area / sheet area, rounded up

Assumptions and defaults

  • Sheet size defaults to 32 square feet, a standard 4x8 sheet. Enter 48 for 4x12.
  • Openings default to 50 square feet, roughly one door and two windows.
  • Waste defaults to 10 percent.
  • Compound and tape quantities are conservative starter amounts, not an exact takeoff.

Concrete Bag vs Ready-Mix Calculator

Formula

  1. Cubic feet = length x width x (thickness in inches / 12)
  2. Adjusted cubic feet = cubic feet x (1 + waste %)
  3. Cubic yards = adjusted cubic feet / 27
  4. 80 lb bags = adjusted cubic feet / 0.6, rounded up
  5. 60 lb bags = adjusted cubic feet / 0.45, rounded up

Assumptions and defaults

  • An 80 lb bag yields 0.6 cubic feet and a 60 lb bag yields 0.45 cubic feet.
  • Waste defaults to 10 percent.
  • The pour is a rectangle of even thickness. Thickened edges and footings need to be added separately.
  • The ready-mix price excludes delivery, short-load, and pump fees.

Fence Material Calculator

Formula

  1. Sections = fence length / post spacing, rounded up
  2. Posts = sections + 1
  3. Rails = sections x rails per section
  4. Pickets = (fence length x 12) / (picket width + gap), plus waste, rounded up
  5. Concrete bags = posts x 2

Assumptions and defaults

  • One straight run. Each corner or end adds posts, and gates need their own posts and hardware.
  • Two bags of concrete per post hole.
  • Pickets default to 5.5 inches wide, the actual width of a nominal 1x6 board.
  • The budget covers posts and pickets only, not rails, concrete, fasteners, or gates.

Deck Material Calculator

Formula

  1. Deck area = length x width
  2. Linear feet of decking = deck area x 12 / (board width + gap)
  3. Waste = waste buffer + 5% for diagonal or picture-frame layouts
  4. Boards = linear feet x (1 + waste %) / board length, rounded up
  5. Joists = (length x 12) / joist spacing, rounded up, plus 1
  6. Fasteners = linear feet x 12 / joist spacing, x 2 screws for wood or x 1 clip for composite

Assumptions and defaults

  • A simple rectangle. Boards are counted by linear feet, so offcuts from one row can finish another.
  • Boards default to 5.5 inches wide, the actual width of 5/4x6, 2x6, and most composite decking.
  • Joists default to 16 inches on center, run across the deck width, and are bought in even lengths.
  • The cost covers deck boards only. Framing, footings, fasteners, railings, and stairs are extra.

Contractor Quote Comparison Calculator

Formula

  1. Quote spread = highest quote - lowest quote
  2. Average quote = sum of quotes / number of quotes
  3. Normalized budget = average quote + allowance adjustment
  4. Planning budget = normalized budget x (1 + contingency %)
  5. Deposit = average quote x deposit %

Assumptions and defaults

  • Quotes entered as zero are ignored, so the tool also works with two bids.
  • The allowance adjustment is a single dollar figure you estimate from the written quotes.
  • Contingency defaults to 12 percent.
  • The tool compares price only. License, insurance, references, and schedule need a separate check.

Roof Shingle Calculator

Formula

  1. Footprint = house length x house width, including overhangs
  2. Roof area = footprint x pitch factor
  3. Adjusted area = roof area x (1 + waste %)
  4. Squares = adjusted area / 100
  5. Bundles = squares x 3, rounded up

Assumptions and defaults

  • Three bundles of shingles cover one roofing square.
  • Pitch factor defaults to 1.12, close to a 6/12 roof.
  • Waste defaults to 15 percent.
  • A simple roof. Dormers, valleys, and multiple levels add area and waste the footprint method cannot see.

Corrections

If a result looks wrong, email hello@projectmath.online with the page URL and the numbers you entered. Use the Copy estimate link button on any calculator to share the exact inputs. Confirmed errors are fixed and the review date on the page is updated.