How the calculators work
Each calculator applies a standard formula to the numbers you enter. This page explains what they do, where the typical values come from, how the results are checked, and what they leave out.
The principles
- Formulas are shown. Every calculator page lists its formula and a worked example, so you can check any result by hand.
- Nothing is sent anywhere. The calculations run in your browser. The numbers you type are not sent to this site.
- Results are estimates. They are meant for planning and for talking to a supplier or lender, not as quotes or advice.
- Examples are labeled. Where a form is pre-filled, the numbers are examples to replace with your own.
Materials calculators
The gravel, pea gravel, crushed stone, sand, topsoil and mulch calculators work out volume as area multiplied by depth, then convert it.
- Volume in cubic feet = length × width × depth in feet (inches divided by 12).
- Cubic yards = cubic feet ÷ 27.
- Extra allowance: volume × (1 + extra percent ÷ 100).
- Weight = cubic yards × tons per cubic yard.
- Bags (mulch) = volume in cubic feet ÷ bag size, rounded up to a whole bag.
Internally the calculators work in metric units and convert, using exact conversion factors (for example, one cubic yard is 0.764554857984 cubic meters). That keeps US and metric answers consistent.
Flooring
The flooring calculator adds a waste allowance to the floor area, divides by the coverage printed on one box, and rounds up to whole boxes. The waste figures on the page come from published flooring guides, which vary, so they are starting points. The calculator also shows how much flooring is left over after laying.
Typical weights
The starting weights are typical figures, in US tons of 2,000 pounds per cubic yard:
| Material | Starting value |
|---|---|
| Gravel (general) | 1.4 |
| Pea gravel | 1.35 |
| Crushed stone | 1.4 |
| Sand | 1.35 |
| Topsoil | 1.1 |
They come from published supplier, landscaping and extension guides, which often disagree with each other, and real weights change with moisture and the exact material. That is why the density box can be edited, and why the pages tell you to ask your supplier.
Property calculators
- Rental property: net operating income is collected rent less vacancy and running costs. Cap rate is that income divided by the purchase price. Cash flow is net operating income less loan payments, and cash-on-cash return is annual cash flow divided by the cash you put in.
- DSCR: monthly rent divided by the full monthly payment on principal, interest, taxes, insurance and association dues. The largest-loan figure works backward from a target ratio.
- House flip: total cost is price, buying costs, repairs with contingency, holding costs, loan costs and selling costs. Net profit is the after repair value less total cost. The 70% rule maximum offer is the after repair value times 0.70, less repairs.
- Loan payments use the standard amortizing loan formula, checked against published payment tables.
These are screening tools. They are before income tax, and they leave out items such as appreciation, one-off repairs after purchase and selling costs on a rental. The pages say what each one leaves out.
How the results are checked
Every formula has automated tests. The expected answers are worked out separately, in a different program from the one that runs on the page, and the two have to agree. There are over a hundred checks covering normal cases, edge cases such as all-cash purchases and blank optional fields, and invalid inputs. Where a published example exists, such as a lender's DSCR example or the standard monthly payment on a $100,000 loan, the calculators are also checked against it.
Checks can show that the maths is right. They cannot show that your inputs are right, or that a real supplier or lender will treat your job the same way.
What they cannot do
- Replace a quote from a supplier, or a figure from a lender.
- Know your local prices, soil, climate, rules or market.
- Give financial, legal, tax or engineering advice.
Found a mistake?
If a result looks wrong, or a figure on a page looks out of date, please get in touch. I will check it and correct it.