Why does my result differ slightly from Excel?

Rounding order, binary float vs decimal.js, and display precision can all differ.

Why are “stress” and “pressure” separate if both are Pa?

Same SI dimension, different engineering intent: use **pressure** for fluid/static operating pressure and **stress** for solid allowable stress, Young’s modulus, or ksi-style material tables—so the UI does not encourage mixing contexts.

Does rpm convert exactly to rad/s?

Yes for the definition used here: ω (rad/s) = rpm × 2π/60. Use rad/s when chaining into formulas that expect SI angular velocity.

Can I pin favorite conversions?

Use browser bookmarks on the routed locale path; state export is optional per feature flags.

Are values logged remotely?

No. Conversion happens locally.

Is Nm³ the same as m³ in this tool?

No. **Nm³** and **Nm³/h** live in their own categories and only convert among **normal** (reference-state) units. **m³** / **m³/h** are separate operational volumes/flows; tying them to Nm³ at other T or p needs density or a gas law outside this linear registry.

In handbooks, is surface-tension kg/m a density?

No. In heat-exchanger tables and SysCAD, **kg/m** and **g/cm** mean **kgf/m** and **gf/cm**—force per length, same dimension as **N/m** (1 kg/m ≈ 9.807 N/m). Water ~72 mN/m is about **7.3 kg/m** in that notation. Do not confuse with **linear mass density**.

Why don’t results match a 1980s handbook conversion table exactly?

Handbook tables were limited to 3–4 significant figures and mixed unit naming (e.g. kg for kgf). This tool uses full-precision NIST/ISO constants and IT calories/BTU; values should agree within handbook rounding, not digit-for-digit with every old table cell.

Engineering unit converter — time, stress, power extensions & FAQ

What this converter covers—and what it does not

Categories are defined in `src/lib/units/registry.ts` with SI anchors and published conversion factors. Recent extensions add **time** (duration in s…d, not civil time zones), **rotational speed** (rad/s anchor; rpm uses 2π/60), **stress / elastic modulus** (same Pa dimension as pressure but a **separate** category for material strength), **MW** and **BTU/h** (1055.05585 J per IT BTU), **mph** / **knots** for speed, and **standard volume / standard volumetric flow** (Nm³, NL, Nm³/h…) referenced to **0 °C, 101325 Pa, dry gas**—kept apart from everyday **m³** / **m³/h** so operating-condition corrections are not implied. Compound or offset-heavy units may still need manual decomposition outside the registry.

Model assumptions

Linear factors only (except temperature, which uses offsets in its own category). Values follow (value × factor_to_SI_anchor) / factor_target.
Rotational speed: rpm and rev/s map to rad/s via 2π; °/s maps via π/180.
Stress category uses the same pascal definition as pressure but is listed separately to avoid mixing **operating pressure** with **material strength** in workflows.
BTU/h uses the same international steam-table BTU (1055.05585 J) as the energy category for consistency.
Speed: mph uses the international mile 1609.344 m; knot uses 1852 m per nautical mile.
Currency, locale/civil timezone rules, and calendar scheduling are not modeled; the **time** category is elapsed duration only.
Import/export JSON is validated locally; malformed files are rejected with translated errors.
**Standard volume / standard flow** (Nm³, NL, Nm³/h, …): factors convert only among units that share the same reference state **T = 0 °C, p = 101325 Pa, dry** (ideal-gas-style bookkeeping). They do **not** replace an equation of state for converting to **actual** m³ or m³/h at line temperature and pressure.
**Surface tension**: in Obana-style handbooks and SysCAD SurfTens, **kg/m** and **g/cm** mean **kgf/m** and **gf/cm** (force/length)—same dimension as **N/m**, not mass per length.

Legacy engineering units & literature lineage

The registry includes **P0/P1 legacy units** commonly found in pre-SI heat-transfer and chemical-engineering handbooks—e.g. kcal/(m²·h·°C), cal/(cm²·s·°C), cal/(cm·s·°C), kcal/(kg·°C), CHU, kg/(m·h), dyn, poundal, erg, kgf·m (work), kcal/s, inHg, lb/gal(US), US short ton, and oil barrel (42 US gal).
Numeric factors are **derived from NIST/ISO definitions** (g = 9.80665 m/s², IT calorie 4.1868 J, IT BTU 1055.05585262 J, 1959 inch definition, etc.) and computed with decimal.js—**not** copied from four-digit tables in older printed references.
Literature lineage: many heat-exchanger workflows still cite **Obana, Heat Exchanger Design Handbook** (1974 JP / 1982 CN translation, Petroleum Industry Press). That appendix motivated unit coverage; the handbook is **not** the numeric SSOT when it disagrees with modern constants.
**Surface tension** now includes handbook/SysCAD units **kg/m (kgf/m), g/cm (gf/cm), N/cm, N/mm, lb/ft**; selecting kg/m shows a one-time note that it is not mass/length.
Temperature deltas in heat-transfer coefficients use **°C or K interchangeably for ΔT** (same numeric increment); anchors remain SI (W/(m²·K), W/(m·K), J/(kg·K)).
Intentionally **excluded** niche units from the same tables: Japanese shaku/kan, Chinese dan (石), the obsolete Chinese glyph for kilowatt (瓩), and duplicate “kg (mass)” labels—use kg, kgf, or Pa·s instead.
When reconciling with Excel or a scanned handbook table, expect small differences: handbook rounding, 15 °C vs thermochemical calories, or mixed kgf/kg force notation in old prose.

References & further reading

BIPM SI Brochure for SI base and derived units.
NIST SP 811 on expressing uncertainty and significant digits (mirrored by the precision slider).
ISO 80000 series for quantities and units—conceptual alignment for stress as pressure-type quantity.
IEC 60050 for rotational machinery vocabulary (rpm as conventional rotation rate).

When the toolbar exposes “Copy shareable link”, inputs can be encoded in the URL so colleagues can reopen the same case. All computations run locally in your browser; this tool may not expose share links yet.

Frequently asked questions

Why does my result differ slightly from Excel?: Rounding order, binary float vs decimal.js, and display precision can all differ.
Why are “stress” and “pressure” separate if both are Pa?: Same SI dimension, different engineering intent: use **pressure** for fluid/static operating pressure and **stress** for solid allowable stress, Young’s modulus, or ksi-style material tables—so the UI does not encourage mixing contexts.
Does rpm convert exactly to rad/s?: Yes for the definition used here: ω (rad/s) = rpm × 2π/60. Use rad/s when chaining into formulas that expect SI angular velocity.
Can I pin favorite conversions?: Use browser bookmarks on the routed locale path; state export is optional per feature flags.
Are values logged remotely?: No. Conversion happens locally.
Is Nm³ the same as m³ in this tool?: No. **Nm³** and **Nm³/h** live in their own categories and only convert among **normal** (reference-state) units. **m³** / **m³/h** are separate operational volumes/flows; tying them to Nm³ at other T or p needs density or a gas law outside this linear registry.
In handbooks, is surface-tension kg/m a density?: No. In heat-exchanger tables and SysCAD, **kg/m** and **g/cm** mean **kgf/m** and **gf/cm**—force per length, same dimension as **N/m** (1 kg/m ≈ 9.807 N/m). Water ~72 mN/m is about **7.3 kg/m** in that notation. Do not confuse with **linear mass density**.
Why don’t results match a 1980s handbook conversion table exactly?: Handbook tables were limited to 3–4 significant figures and mixed unit naming (e.g. kg for kgf). This tool uses full-precision NIST/ISO constants and IT calories/BTU; values should agree within handbook rounding, not digit-for-digit with every old table cell.

Extended copy on this page (headings, assumptions, references, FAQs) may be drafted or localized with AI assistance; engineering judgment and governing codes still apply. Numerical models run locally in your browser as implemented. For contract-critical work, cite primary standards and qualified review.