How WashTally turns inputs into a pricing scenario.

One versioned model, five different questions.

Model version 1.0.0 defines gutter adjustment ordering, general rectangular area, driveway trapezoid-plus-apron area, a fee-aware minimum price, and profit/margin/markup. The public formula register links each question to its scenario and expected output.

The minimum tool starts with costs and solves for a price. The profit tool starts with known revenue and subtracts costs. The two surface tools share pricing arithmetic, but driveway measurement and its separate edge-time input perform a distinct job.

Input values are not market-price evidence.

Live rate fields start empty. Optional cost/extras fields start at zero so that omissions remain the visitor’s responsibility; no zero is claimed to be the true cost of running a business. The worker-count field starts at one as a count, not a labour-cost assumption.

Worked examples use explicitly hypothetical rates, durations and cost allocations. Search data helped select page topics; it does not substantiate price presets, wages, cleaning speeds or safe work methods. No universal regional rate is claimed.

What the arithmetic checks cover.

Automated tests compare declared scenarios with fixed expected outputs and check invalid dimensions, guard length, worker/downspout counts, fee/margin denominators, zero-revenue ratios, discounts, negative profit and unit equivalence. These are mathematical and interface checks.

They do not represent professional trade review, field trials, property inspection, accounting advice or certification. Public claims of those kinds remain unsupported.

Units, money and rounding are explicit.

The length factor is 0.3048 metre per international foot; the area factor is its square. Existing measurement fields and matching length/area rates convert together when the browser unit control changes. Currency selection changes formatting only and warns that numbers have not been exchanged.

Customer adjustment rows are rounded to cents before addition. Fee-aware floor/target calculations round up to cents. Ratios are displayed to two decimals; a zero denominator is unavailable. Taxes are excluded and no tax treatment is chosen.

An optional browser enhancement; a manual path underneath.

Navigation, guide text, formulas, source records, examples and manual worksheets are rendered as HTML. The small calculator enhancement uses the shared model in the browser; it makes no fetch request and does not write calculator inputs to browser storage.

Without JavaScript, automatic calculation controls are absent and the visible GBP/metres worksheet, formula and fixed example support manual arithmetic. No automatic output is claimed in that state.

Correction handling has an operational gate.

There is no active public feedback channel on this website. Do not send customer addresses or property details through an unapproved route. A monitored channel and its handling process must be established before Contact is enabled.

Model changes should update the version, expected-output records and affected examples. No fixed review schedule or reply-time promise is invented here.

Illustrative arithmetic / not market rates

What a test result establishes

The gutter scenario has a fixed expected £190 charge and £119.50 included costs.

A passing arithmetic test demonstrates that the implemented calculation matches the documented scenario; it does not validate the real-world assumptions.

Scenario record checked 14 October 2026. Read the formula and scenario register.

Methodology: questions