Methodology
How measurements and calculations are handled
Inputs, conversion, formulas, and presentation are separate steps. That separation makes a result easier to test, explain, and use safely.
1. Start from an explicit unit system
A selected metric input accepts millimetres by default and explicit mm, cm, or
m suffixes. A selected imperial input accepts decimal inches, simple or mixed fractions, and
feet-plus-inches notation. A decimal point is always a full stop and grouped values such as
1,200 are rejected rather than guessed.
2. Normalize once
Valid physical lengths become a typed decimal-millimetre value at the calculation boundary. One inch is exactly 25.4 millimetres and one foot is exactly 304.8 millimetres. Raw text, physical measurements, and future drawing coordinates remain different types.
3. Keep calculated and displayed values distinct
Conversion and intermediate formula values are not rounded. A precise display supports checking, while a separately labelled practical display may round to a site-readable millimetre or fractional inch. Totals are calculated before presentation rounding.
4. Validate before calculating
Each tool checks syntax, units, scalar ranges, relationships, and physical feasibility in that order. An impossible layout must return an actionable error instead of an ordinary-looking result or preview.
5. Test invariants and worked examples
Tests cover metric and imperial equivalents, malformed and boundary values, conversion, rounding midpoints, exact-fit geometry, and reconstruction of the original usable dimension. Feature formulas are versioned when implemented.
Planning limitation
Verify site dimensions, actual material sizes, wall conditions, manufacturer instructions, applicable codes, and qualified trade advice. A proportional diagram is explanatory and is not a printable template.