Calculation Methodology
How every calculator on this site actually computes its answer — and why the answer is a range.
Last updated: July 2026
The principle: show the working
A calculator that hides its formula is asking for trust it has not earned. Every calculator here publishes, on its own page, the formula it applies, the assumptions behind it, the wastage allowances built in, and at least one fully worked example. You should be able to put the site aside, take a pen, and arrive at the same answer. If you can't, either our page is unclear or our code is wrong — both are bugs, and both are worth reporting.
Where the arithmetic runs
Calculations execute in your browser, in plain JavaScript, on your own device. Nothing is sent to a server — see the Privacy Policy. Each calculator's logic is a small, purpose-written function rather than a formula string interpreted at runtime; this is both a security property and a discipline, because a formula that must be written as real code cannot quietly disagree with the one printed on the page.
The rate data those functions read is generated from the same source file that renders the rate tables you see. A calculator physically cannot use a number different from the one published beside it.
Why answers come out as ranges
Our inputs are ranges, so our outputs are ranges. When a calculation combines them, lows are combined with lows and highs with highs — a 1,000 sq ft house at a ₹1,500–2,500 per sq ft standard-build band gives ₹15,00,000 to ₹25,00,000, not a single averaged figure.
Averaging the band into one number would be more comfortable to read and less useful to act on. The width of the range is the information: it tells you how much your finish choices, your region and your season actually matter.
The cost split
Whole-house estimates are broken into three buckets. Published sources quote roughly 55–60 / 30–35 / 10–15; we use the normalised midpoints below, which sum to exactly 100%.
| Bucket | Share of construction cost | What it covers |
|---|---|---|
| Structure & masonry | 57% | Foundation, RCC frame, masonry, plaster |
| Finishing | 31% | Flooring, painting, doors and windows, kitchen and bath surfaces |
| Services | 12% | Electrical, plumbing, sanitary |
The split applies to construction cost only. Land, boundary wall, external development, modular interiors, professional fees and contingency all sit outside it — see the exclusions below.
Wastage: counted once, never twice
Site wastage is real and every honest estimate includes it. The trap is counting it twice, which inflates a material order by a tenth for no reason. Our rule is explicit:
- Per-square-foot thumb rules already include wastage. Where a calculator estimates cement, steel, brick or sand from built-up area, the published quantity band has typical site wastage baked in. Do not add more on top.
- Calculators that start from a base quantity add wastage explicitly, using the shared allowances below, and show it in the working.
| Material | Wastage allowance |
|---|---|
| Cement | 3% |
| Steel | 5% |
| Bricks | 5% |
| Sand | 10% |
| Aggregate | 5% |
| Paint | 10% |
| Tiles | 10% |
Each calculator page states which of the two paths it takes. It is never both.
Units
Unit confusion causes more estimating errors than arithmetic does. We state the unit on every rate and never mix systems silently. Watch these in particular when comparing a supplier's quote against our figures: a cement bag is 50 kg; sand and aggregate are quoted per cft here, while suppliers may quote per tonne, per brass (100 cft) or per truckload; bricks are priced per 1,000 numbers; steel is per kg, while mills quote per tonne. Always ask a supplier which unit their number is in before comparing it with ours.
Rounding
We round only at the point of display, never mid-calculation, so rounding never compounds. Quantities are shown as whole units — you cannot buy 0.3 of a brick — and money is shown in whole rupees or in lakhs where a figure would otherwise be unreadably long. The rounded figure may differ from a hand calculation by a rupee or two; the underlying working is exact.
What our estimates exclude
Unless a calculator page says otherwise, construction-cost figures exclude:
- Land, registration, stamp duty and legal costs
- Approvals, sanction fees and utility connections
- Compound wall, gate, landscaping and external development
- Modular interiors — kitchen, wardrobes, furniture
- Architect and structural consultant fees, typically 2–5%
- Contingency, for which 5–10% is the conventional allowance
They are excluded because they vary far too much to model responsibly, not because they are small. Budget for them separately.
Where our models come from
Formulas are built on established engineering practice — IS codes, CPWD norms and the thumb rules used in Indian site practice — then simplified for someone planning a house rather than detailing one. Where a thumb rule genuinely does not fit a build type, we say so on the page instead of hiding it: AAC-block construction, for instance, needs roughly 15–20% less cement and mortar than conventional brick masonry, and a partition-heavy layout in 4.5-inch walls changes the brick count substantially.
The rate side of the model is documented separately under How We Source Rates.
Verification
Before a calculator is published, its worked examples are re-run through the live tool to confirm they reproduce the numbers printed on the page, its units are checked explicitly, and its published formula is compared against the code path that actually executes. The site build additionally refuses to compile if a page's stated wastage allowance disagrees with the shared cost model — the class of drift that would otherwise be invisible.
Limits
These are budgeting models, not designs. They cannot see your soil report, your span layout, your seismic zone or your site access, and they are not a substitute for a structural engineer or a proper bill of quantities. The Disclaimer sets out the boundary in full, and each calculator page lists what that specific tool does not model.