Numbers & Units
| Rule | Do | Don't |
|---|---|---|
| Words vs numerals (running text) | spell out zero–nine; numerals for 10+ | "5 reasons", "twelve teams" |
| With a unit or in data, always numerals | 5 kg, 3 laps, 0 °C | five kg |
| Decimal separator (international/technical) | point: 3.14 | 3,14 (comma — German locale only) |
| Thousands grouping | thin space: 10 000, 1 000 000 | 10,000 / 10.000 (locale-ambiguous) |
| Values below 1 | leading zero: 0.5 | .5 |
| Ranges | en dash, no spaces: 10–20; or "10 to 20" | 10-20 (hyphen), 10 – 20 |
| Negative numbers | minus sign: −5 | -5 (hyphen) |
| Percent | 45 % (narrow no-break space, per SI) | 45% (no space); 45 % (breakable space) |
| Dates & time | ISO: 2026-07-21, 24-hour 14:30 | 07/21/26, 2:30 pm |
Note: German-language documents use a comma decimal (3,14). Never mix conventions within one document.
| Rule | Do | Don't |
|---|---|---|
| Space between value and unit a protected (non-breaking) space | 5 kg, 100 km/h, 20 °C (never wraps) | 5kg (no space) "5 kg" with an ordinary, breakable space |
| Exception: plane-angle degree | 90° (no space) | 90 ° |
| Symbols are case-sensitive | kg, m, s, A, K, Hz, W, V, Pa | Kg, KM, sec, hz |
| Units from names: symbol upper, word lower | 5 N (newton), 10 W (watt) | 5 Newton, 10 w |
| Don't pluralise or add periods to symbols | 5 kg | 5 kgs, 5 kg. |
| One prefix only | µm, kN, MPa | mµm, kkg |
| Litre uses capital L | 5 L, 250 mL | 5 l (looks like 1) |
The gap between a value and its unit (and before %) must be a
protected space so the two never split across a line break —
ideally a narrow no-break space (U+202F,  ).
Fall back to a normal no-break space ( ) where the narrow one
isn't available. Never use an ordinary space, which can wrap.
Equations are authored in LaTeX and diagrams / technical figures in
TikZ, then exported as vector graphics (SVG/PDF) and embedded as images.
Do not hand-build equations in HTML (<sup>/<sub>)
or with client-side renderers (KaTeX/MathJax). Prefer the siunitx package for
numbers and units — it applies the correct thin protected space automatically.
| Rule | Do (LaTeX) | Don't |
|---|---|---|
| Numbers & units via siunitx | \qty{5}{\meter\per\second}, \num{10000} | 5 m/s with manual spacing |
| Proper operators | \times, \cdot, \le \ge \ne \pm \approx | x, ASCII <=, != |
| Variables in math mode (auto-italic) | $v$, $a_{x}$ | upright text-mode variables |
| Vectors bold / arrow | \mathbf{F} or \vec{F} | plain F (ambiguous) |
| Exponents & subscripts (brace multi-char) | v^{2}, t_{2}, E_{\mathrm{kin}} | v^2 (unbraced), E_kin |
| Fractions | \frac{a}{b+c} | a/b+c |
| Display & numbering | equation / align (auto-numbered) | manual "(1)"; inline for long derivations |
| Upright roman functions / differentials | \sin, \log, \mathrm{d}x | sin, italic d |
LaTeX source examples:
v = \frac{s}{t}
E_{\mathrm{kin}} = \tfrac{1}{2}\, m v^{2}
a = \frac{v_{2} - v_{1}}{t_{2} - t_{1}}
% number with unit (siunitx) — handles the protected thin space automatically:
\qty{9.81}{\meter\per\second\squared}Equations render to vector (SVG/PDF) through the LaTeX toolchain and are embedded as images. The Symbol Quick Reference below is for one-off symbols in running text / UI copy, not for equations — those always go through LaTeX.
| Symbol | Meaning | HTML entity | Example |
|---|---|---|---|
| × | multiplication | × | 5 × 3 |
| − | minus | − | −5 °C |
| · | product / middle dot | · | N·m |
| ± | plus–minus | ± | 10 ± 0.5 |
| ≤ ≥ | less/greater or equal | ≤ ≥ | x ≤ 10 |
| ≠ | not equal | ≠ | a ≠ b |
| ≈ | approximately | ≈ | π ≈ 3.14 |
| ° | degree | ° | 90°, 20 °C |
| µ | micro prefix | µ | 5 µm |
| narrow no-break space (value ↔ unit, %) |   (U+202F) | 5 kg, 45 % | |
| no-break space (fallback) | (U+00A0) | 5 kg | |
| ² ³ | superscript 2 / 3 | ² ³ | m², m³ |
| ½ | one half | ½ | ½ m v² |
| Δ | delta (change) | Δ | Δt |
| √ | square root | √ | √2 |
| ∑ | sum | ∑ | ∑ F |