Methodology and sources

The principle

A calculator is only useful if you can trust the answer, and you can only trust an answer you are able to check. So every calculator on this site follows the same rules:

1. The formula is printed on the page, in words and symbols, so that any result can be reproduced with a pencil.
2. Every assumption is listed in a table on the same page, with the value used and the reason for it.
3. The result is broken down line by line: area, volume, allowance, total, and how it was rounded.
4. Assumptions that vary between products or suppliers can be replaced with your own figure.

How a result is built

All of the calculators work through the same four stages.

Stage 1: Geometry. Your measurements are turned into an area, a length or a volume. A rectangle is length times width. A circle is 3.1416 times the radius squared. A triangle is base times height, divided by two. A volume is an area times a depth. These are exact, and the only error is in the measurements themselves.

Stage 2: Allowance. A percentage is added for the losses that the geometry cannot see: uneven ground, settling, compaction, cutting waste, breakage. Each calculator has a default allowance chosen for its material, and you can change it. Compaction of base gravel is treated as a separate step from general waste, because it is predictable and large. The reasoning behind each default is in our guide to waste factors.

Stage 3: Conversion. The quantity is converted into the units the material is sold in. Volume becomes bags, using the bag size. Volume becomes tons, using a density. Area becomes pounds of seed or fertilizer, using a rate. Length becomes posts, boards or blocks, using a spacing or a unit size. This is the stage that depends on assumptions.

Stage 4: Rounding. The result is rounded to what can actually be bought.

Rounding rules

Rounding is always upward. Being slightly over costs a little money. Being slightly short costs a second trip, a second delivery fee, or a visible gap in the finished job.

Where rounding makes a noticeable difference, the breakdown table shows the figure before and after, so you can see how much of the order is rounding.

How the assumptions are chosen

Assumptions are the values a calculator needs that are not in your measurements. They fall into three kinds, and each is handled differently.

Definitions. A cubic yard is 27 cubic feet. A foot is exactly 0.3048 meters. A pound is exactly 0.45359237 kilograms. These are fixed, and conversions use the exact values so that metric and imperial results agree.

Product facts. A standard 80 pound bag of concrete mix makes 0.60 cubic feet. A 2 × 6 board is 5½ inches wide. A common sod roll is 2 feet by 5 feet. These come from manufacturers’ labels and published product data. They are stable, but they do vary by brand and country, so each calculator states the value it uses and offers the common alternatives.

Typical values. What a cubic yard of crushed stone weighs, how much seed a lawn needs, how much a soil mix settles. These genuinely vary. For each one we take the middle of the range that appears across manufacturers’ data, university extension publications and industry installation guides, and we say plainly that it is a typical figure. Where the variation is large enough to change an order, the calculator lets you enter your own value, and the page tells you to prefer the figure from your supplier or your product label.

We do not use a value we cannot explain. If the reason for an assumption cannot be written in one line of a table, it does not go into a calculator.

Sources

The assumption tables draw on the following kinds of source.

• Product labels and manufacturers’ data sheets, for bag yields, package sizes, paver and block dimensions, and seeding and application rates printed on products.
• University cooperative extension publications, for lawn seeding rates, fertilizer rates and practice, soil amendment and mulch depths. Extension services are the public, research-based advisory arms of land-grant universities in the United States.
• Industry installation guidance, including the published guides of paver and segmental wall manufacturers and of the Concrete Masonry and Hardscapes Association, for base depths, bedding sand, edge restraint, buried courses and drainage behind walls.
• Standard dimensional lumber sizes, as published by lumber grading and standards bodies, for actual board dimensions.
• Basic geometry and unit definitions, for everything else.

We describe typical practice from these sources in our own words. We do not reproduce their tables, and we do not present any single source’s figure as the only correct one. Where sources disagree, we say so and give the range.

Checking

Before a calculator is published, and again whenever its code or assumptions change:

1. Two worked examples are calculated by hand from the printed formula.
2. The same inputs are run through the calculator.
3. Every line of the breakdown must match the hand calculation.
4. One example is repeated in metric, and the result must agree with the imperial one after conversion.
5. The page text is checked against the calculator, so that the numbers quoted in the examples and the FAQ are the numbers the calculator produces.

Guides are checked the same way. Any figure in a guide that could be produced by a calculator is checked against it.

Review schedule

Assumption tables are reviewed every three months, and whenever a reader reports a problem. Each calculator and reference page shows the date of its last review. A review asks three questions. Are the product facts still current? Do the typical values still sit in the middle of the published range? Has anyone reported a result that did not match what they found in practice?

Changes are recorded in the corrections log with the date and what changed.

What the estimates cannot do

These calculators estimate quantities of material. It is important to be clear about what that leaves out.

• They are not structural design. The deck, fence, retaining wall and concrete calculators count materials for dimensions you supply. They do not check that those dimensions are safe or lawful. Spans, footing sizes, post depths below frost, wall reinforcement and anything requiring a permit must come from your local building code, the manufacturer’s instructions, or an engineer.
• They do not know your site. Soil type, slope, drainage, frost depth and access all change how a project should be built.
• They do not know your product. A bag, a block or a seed blend can differ from the typical values used. The label on your product takes priority over any figure here.
• They depend on your measurements. An area that is wrong by 20% gives an order that is wrong by 20%. Our guide to measuring irregular beds is the place to start.
• Prices are yours. The cost comparisons use only the prices you type in. The site does not collect or publish prices, and example prices in the guides are illustrations of a method, not quotes.

Treat every result as a well-reasoned estimate to order from, not as a guarantee. The full terms are in the disclaimer.

Privacy of what you enter

The calculators run entirely in your browser. The measurements and prices you type are not sent to us and are not stored. A “Copy link to this result” button puts your inputs into a link so that you can save or share it, and only someone you give that link to can see them.

Tell us if something is wrong

If a result does not match what you found on a real project, that is exactly what we want to hear about. Send the link to your result, and what happened, to contact@thewovengarden.com or through the contact page.