Methodology: How Our Calculators Work
Last updated
Every calculator on this site shows its work: the breakdown under each result lists every step from your measurements to the final number. This page explains the rules behind those steps in one place: the formulas, the material weights and bag sizes we assume, how waste and rounding work, what the estimates leave out, and how we test them. It ends with a log of changes to the math.
The five steps behind every bulk-material estimate
The gravel, driveway, concrete, mulch, and topsoil calculators share one volume model:
- Area. Worked out from the shape you choose, using the formulas below.
- Volume. Area × depth, with the depth converted from inches to feet (÷ 12). Cubic feet ÷ 27 = cubic yards.
- Waste allowance. Volume × (1 + waste %). This waste-adjusted volume is the amount you actually buy.
- Weight, bags, and cost. All three come from the waste-adjusted volume: weight is cubic yards × the material's weight per cubic yard, bags are cubic feet ÷ the bag size (rounded up), and cost is tons × your price per ton (cubic yards × your price per cubic yard for concrete). Cost appears on the gravel calculator, and on the driveway and concrete calculators when you enter an optional price.
- Suggested order. The waste-adjusted volume, rounded up to the increment suppliers sell: 0.1 yd³ for bulk material and 0.25 yd³ for ready-mix concrete.
Worked example
Here is the Gravel Calculator's own breakdown for a 20 × 10 ft area of crushed stone (#57), 4 in deep, with 10% waste at $55 per ton. These are the same rows you see under the result on the calculator:
| Step | Result |
|---|---|
| Area · Rectangle / Square | 200 ft² |
| Volume (200 × 0.33 ft) | 66.7 ft³ |
| Required (converted to yd³) | 2.47 yd³ |
| With 10% waste | 2.72 yd³ |
| Suggested order | 2.8 yd³ |
| Weight (Crushed Stone (#57), ~2600 lb/yd³, incl. waste) | 3.53 tons |
| Bags at 0.5 ft³ each (incl. waste) | 147 bags |
| Cost (3.53 tons × $55.00/ton) | $194.20 |
Why weight and cost ignore the rounded order
The suggested order is a purchasing figure, not a measurement. If weight and cost were calculated from it, the tons you pay for wouldn't match the volume and bag count for the same job, and a tiny change in your measurements could make the cost jump by a whole rounding step. Keeping every figure on the same waste-adjusted volume means the numbers always agree with each other. Early versions of the Gravel and Topsoil calculators mixed the two; see the change log below.
Area formulas
The gravel, driveway, concrete, mulch, topsoil, paver, and sod calculators accept five shapes, and the breakdown shows the formula used.
| Shape | Formula | What you measure |
|---|---|---|
| Rectangle or square | length × width | Length and width |
| Circle | π × radius², where radius = diameter ÷ 2 | Diameter at the widest point |
| Triangle | ½ × base × height | Base, and the perpendicular height from it |
| Circular ring | π × (outer radius² − inner radius²) | Outer and inner diameters, e.g. a path around a bed |
| Trapezoid | ½ × (side A + side B) × width | Both parallel sides and the distance between them |
For an irregular area, split it into these shapes and add the results. On a slope, measure along the ground rather than the level distance, since the material covers the sloped surface.
Material weights and bag sizes
Weight turns a volume into tons, which is how many suppliers price bulk material. These are the exact figures each calculator uses:
| Material | Weight per cubic yard | Tons per cubic yard | Bag size |
|---|---|---|---|
| Gravel: Crushed Stone (#57) | 2,600 lb | 1.3 | 0.5 ft³ |
| Gravel: River Rock | 2,650 lb | 1.325 | 0.5 ft³ |
| Gravel: Crushed Limestone | 2,700 lb | 1.35 | 0.5 ft³ |
| Gravel: Pea Gravel | 2,800 lb | 1.4 | 0.5 ft³ |
| Gravel: Decomposed Granite | 2,800 lb | 1.4 | 0.5 ft³ |
| Driveway base and surface stone | 2,800 lb | 1.4 | — |
| Concrete | 4,050 lb (150 lb/ft³) | 2.025 | 80 lb bag makes 0.6 ft³ |
| Topsoil (moist, screened) | 2,200 lb | 1.1 | — |
| Mulch (shredded bark) | 500 lb | 0.25 | 2 ft³ |
- Driveway stone is listed heavier than #57 because base and surface layers usually contain fines, as in crusher run or #411, which fill the gaps and pack denser.
- Real weights vary with the stone, its grading, and especially moisture. If your supplier quotes a different weight per yard, scale our tonnage by the ratio: at 2,900 lb per yard instead of 2,600, multiply by 2,900 ÷ 2,600 ≈ 1.12.
- Concrete uses 150 lb per cubic foot, the usual figure for normal-weight concrete, and bag yields follow manufacturers' published figures.
- Mulch and topsoil weights swing the most with moisture, so treat them as a guide for hauling rather than an exact figure.
Waste allowances
A waste allowance covers what real projects lose: material that settles and compacts, dips in the ground, spills, and offcuts. Each calculator offers a range, with a default that suits typical jobs.
| Calculator | Options | Default | What it covers |
|---|---|---|---|
| Gravel Calculator | 5%, 10%, 15% | 10% | Covers uneven sub-grade, spillage, and compaction. 10% works for most driveways. |
| Concrete Calculator | 5%, 10%, 15% | 10% | Covers spillage, uneven forms, and over-excavation. 10% works for most slabs. |
| Mulch Calculator | 5%, 10%, 15% | 10% | Covers settling, uneven ground, and spillage. 10% works for most garden beds. |
| Topsoil Calculator | 5%, 10%, 15% | 10% | Covers settling and uneven grading. 10% works for most beds and lawns. |
| Paver Calculator | 5%, 10%, 15% | 10% | Covers cuts, breakage, and edge trimming. 10% works for most simple layouts. |
| Sod Calculator | 5%, 10% | 5% | Covers curves, obstacles, and edge trimming. 5% works for most rectangular lawns. |
| Fence Calculator | 5%, 10% | 5% | Covers cutting waste and damaged rail pieces. 5% works for most standard runs. |
| Paint Calculator | 0%, 10% | 10% | Covers cutting-in, touch-ups, and second-coat overlap. 10% works for most rooms. |
| Deck Calculator | 10%, 15% | 10% | Covers angled cuts, staggered seams, and defects. 10% works for most simple decks. |
| Driveway Calculator | 5%, 10%, 15% | 10% | Covers compaction, spillage, and uneven sub-grade. 10% works for most driveways. |
Go higher on soft or uneven ground, on curved or angled layouts, and with patterns that need many cuts, such as herringbone pavers. Running short usually costs more than ordering a little extra: a second bulk delivery comes with its own fee, and a second batch of pavers or paint may not match the first.
Rounding rules
Rounding always comes last, and it only affects the quantity to order. The unrounded, waste-adjusted figure drives everything else.
| What | Rounded | Why |
|---|---|---|
| Bulk orders: gravel, driveway, mulch, topsoil | Up to the nearest 0.1 yd³ | Many suppliers sell partial yards; some sell only half or whole yards, so check |
| Ready-mix concrete orders | Up to the nearest 0.25 yd³ | Ready-mix is batched and billed in quarter-yard steps |
| Bags of gravel, concrete, or mulch | Up to a whole bag | You can't buy part of a bag |
| Pavers, sod, fence rails, deck boards | Up to a whole unit | The waste allowance, not rounding, covers cuts |
| Paint | Up to the nearest quart (0.25 gal) | Paint is sold by the quart and the gallon |
| Fence sections | Up to a whole section | The built fence may run slightly past your measured length |
The rounding code also guards against computer arithmetic errors, so an order of exactly 2.2 yd³ stays 2.2 instead of creeping up to 2.3.
Count-based calculators
The paver, sod, fence, paint, and deck calculators count units instead of volume, but follow the same order: measure, add waste, then round up to whole units.
| Calculator | How the count works | Assumptions |
|---|---|---|
| Pavers | Area ÷ the face area of one paver, plus waste, rounded up | Ignores the narrow sand joints, so counts lean slightly high; no border course |
| Sod | Area plus waste, ÷ 10 ft² per roll or 450 ft² per pallet, rounded up | Roll and pallet sizes vary by sod farm |
| Fence | Sections = length ÷ post spacing, rounded up; posts = sections + 1; rails = sections × rails per section, plus waste | Pickets aren't estimated, since their width and spacing vary by style |
| Paint | Wall length × height × coats, plus waste, ÷ 350 ft² per gallon, rounded up to a quart | Doors and windows aren't subtracted; take off about 20 ft² per door and 15 ft² per window |
| Deck | Area ÷ board width = linear feet, plus waste, in 12 ft boards; joists every 16 in across the width, plus one | Ignores gaps between boards; check framing against span tables |
Unit conversions
| Conversion | Multiply or divide by |
|---|---|
| Inches of depth to feet | ÷ 12 |
| Cubic feet to cubic yards | ÷ 27 |
| Cubic yards to cubic meters | × 0.7646 |
| Square yards to square feet | × 9 |
| Square feet to square inches | × 144 |
| US tons to pounds | × 2,000 |
| Pounds to kilograms | × 0.4536 |
| Metric tonnes to US tons | × 1.1023 |
Tons are US short tons (2,000 lb). Metric results are shown alongside the imperial ones where a calculator supports them.
What the estimates leave out
- Excavation. Digging out soil to make room for a base isn't included, and soil takes up more room once it's dug out than it did in the ground.
- Extra compaction. Very soft ground, or thick base layers compacted hard, can use more material than the largest waste allowance.
- Uneven depth. The calculators assume a uniform depth. Thickened slab edges, tapered layers, or filling low spots need to be added separately.
- Supplier specifics. Minimum orders, delivery fees, truck capacity, sales tax, and the exact yield of the bags you buy.
- Structural requirements. Slab reinforcement and footings, deck framing and spans, and fence post depth depend on local codes and site conditions.
See the disclaimer for more on using these estimates, and confirm quantities with your supplier before ordering.
How we check the calculators
- Automated tests cover the shared volume model and every bulk-material calculator. They check that cubic yards equal cubic feet ÷ 27, that the waste-adjusted volume equals the volume × (1 + waste %), that the suggested order is never smaller than what you need, and that weight, bags, and cost all come from the same waste-adjusted volume. The Gravel Calculator's tests run across every area shape, gravel type, and waste setting.
- Rounding has its own tests, including the edge cases where computer arithmetic could push an exact figure up a step.
- One set of numbers. The tables in our guides and on this page are generated from the same constants the calculators use, so they can't quote a different weight or bag size than the calculators do.
- Corrections are logged. When we find a mistake, we fix it and record it in the change log below.
Calculator-by-calculator details
Each calculator explains its specific formula under "How this is calculated":
- Gravel Calculator
Calculate tons, cubic yards and cost for any area.
- Concrete Calculator
Calculate concrete for slabs, footings and more.
- Mulch Calculator
Find out how much mulch you need.
- Topsoil Calculator
Estimate topsoil volume for beds and lawns.
- Paver Calculator
Plan pavers for patios and walkways.
- Sod Calculator
Work out sod rolls or pallets for your lawn.
- Fence Calculator
Estimate posts, rails, and panels needed.
- Paint Calculator
Calculate paint cans for walls and rooms.
- Deck Calculator
Plan decking boards and materials.
- Driveway Calculator
Estimate gravel for your driveway.
Change log
Changes to how the calculators compute their results, newest first.
- Driveway and Concrete calculators: added an optional price field (price per ton for the driveway, price per cubic yard for concrete). When you enter a price, the headline result becomes the material cost, calculated from the waste-adjusted quantity and not the rounded order. With the price left blank, results are unchanged. Cost covers the material only; delivery, taxes, and short-load fees are not included. Automated tests were added for both paths.
- Driveway Calculator FAQ: corrected the recommended crown from 1 in per foot (about 8%) to 1/2 in per foot (about 4%), in line with gravel road maintenance manuals. The gravel cost guide's example now prices the waste-adjusted volume, as the calculator does, and guide tables are generated from the calculators' own constants.
- Weight, bags, and cost now all come from the waste-adjusted volume, never from the rounded order. Previously the Gravel Calculator took weight and cost from a rounded whole-yard order, and the Topsoil Calculator took weight from its rounded order. Bulk orders now round up to the nearest 0.1 yd³ instead of whole yards; concrete keeps quarter-yard rounding. Automated tests were added for these rules.
- Corrected two worked examples that didn't match their calculators: the Driveway Calculator's (it said 3.33 yd³ where the formula gives 11.11 yd³) and the Paint Calculator's rounding wording.
- Added circle, triangle, circular ring, and trapezoid shapes to the gravel, driveway, concrete, mulch, topsoil, paver, and sod calculators.
- Gravel Calculator: added a gravel type setting with per-type weights of 2,600–2,800 lb per cubic yard, replacing a single 2,800 lb figure, and a price per ton so it can estimate total cost.