About Density Converter — kg/m³, g/cm³, lb/ft³ & lb/gal
The density converter translates a density between the units engineering actually mixes on one job: kg/m³ from SI material tables and structural loads, g/cm³ (identically g/mL) from chemistry data and alloy datasheets, lb/ft³ from US construction and soil mechanics, lb/US gal from drilling fluids and coatings, lb/in³ from US machining stock tables, and slug/ft³ from US fluid dynamics where Newton's second law wants consistent mass units. Every conversion runs through the SI unit — the kilogram per cubic meter — using the factors published in NIST SP 811 Appendix B, so a chain like lb/ft³ → kg/m³ → lb/ft³ returns exactly where it started.
The anchors worth memorizing: water sits near 1,000 kg/m³ = 1 g/cm³ ≈ 62.428 lb/ft³ ≈ 8.345 lb/gal (exactly 1,000 kg/m³ only at ~4 °C; 62.4 lb/ft³ is the customary round figure), structural steel at 7,850 kg/m³ ≈ 490 lb/ft³, aluminum at 2,700 kg/m³ ≈ 169 lb/ft³, and normal-weight concrete near 2,400 kg/m³ ≈ 150 lb/ft³. The g/cm³ scale is the same number as specific gravity referenced to water — which is why it is the quickest sanity check in the set.
How It Works
- Enter the density, pick the unit it is currently in ("from"), and pick the unit you need ("to"). The headline shows the converted value; the table below it shows the same density in all seven units at once, so you can lift whichever one the next document needs.
- Every conversion is value × (kg/m³ per from-unit) ÷ (kg/m³ per to-unit). The factors are the NIST SP 811 Appendix B values — g/cm³ and g/mL are exact (the milliliter has been exactly 1 cm³ since 1964); the pound-based factors carry NIST's published 7 significant figures.
- Displayed values are rounded to 6 significant digits. That is display rounding only — the underlying conversion is done in full double precision, which is why round-trips return your original number.
- Watch the gallon: lb/gal here means the US liquid gallon (231 in³). An imperial-gallon density is about 20% larger per gallon, so a UK "10 lb/gal" mud is not the US one. And slug/ft³ exists because US dynamics needs F = ma without conversion constants: divide lb/ft³ by g = 32.174 ft/s² to get it, or let the converter do the equivalent through SI.
Worked Example
A geotechnical report lists saturated soil at 62.4 lb/ft³-referenced water density and the SI calculation wants kg/m³. Through the base unit: 62.4 lb/ft³ × 16.01846 kg/m³-per-lb/ft³ = 999.552 kg/m³ — the familiar "water = 1,000 kg/m³" to within 0.05%, because 62.4 lb/ft³ is itself a rounded customary figure. The same density is 0.999552 g/cm³ and 8.34167 lb/US gal.
Material density chart — kg/m³ to lb/ft³ and g/cm³
Reference densities for the materials engineers reach for first, converted with the NIST SP 811 factor (1 lb/ft³ = 16.01846 kg/m³) and shown to 6 significant digits — exactly what the converter outputs for the same inputs.
| Material (kg/m³) | lb/ft³ | g/cm³ |
|---|---|---|
| Water — 1,000 kg/m³ | 62.428 | 1 |
| Steel — 7,850 kg/m³ | 490.06 | 7.85 |
| Aluminum — 2,700 kg/m³ | 168.556 | 2.7 |
| Concrete — 2,400 kg/m³ | 149.827 | 2.4 |
Formulas
- Conversion through the base unit
x = value × f_from ÷ f_to- kg/m³ per unit (the factor table)
g/cm³ 10³ · g/mL 10³ · lb/ft³ 16.01846 · lb/US gal 119.8264 · lb/in³ 27,679.90 · slug/ft³ 515.3788- The everyday anchors
water ≈ 1,000 kg/m³ = 1 g/cm³ ≈ 62.428 lb/ft³ ≈ 8.345 lb/gal; slug/ft³ = (lb/ft³) ÷ 32.174
Standards & References
- NIST SP 811, Guide for the Use of the International System of Units, Appendix B.9 (Density) — pound per cubic foot 1.601846×10¹ kg/m³; pound per gallon (US) 1.198264×10² kg/m³; pound per cubic inch 2.767990×10⁴ kg/m³; slug per cubic foot 5.153788×10² kg/m³; gram per cubic centimeter 10³ kg/m³ (exact)
- The US liquid gallon = 231 in³ exactly (NIST Handbook 44 App. C) — the basis of the lb/gal factor
- Displayed to 6 significant digits; conversions computed in full double precision
Frequently Asked Questions
How do I convert lb/ft³ to kg/m³?
Multiply by 16.01846 (the NIST SP 811 factor). So 62.4 lb/ft³ ≈ 999.55 kg/m³ and 150 lb/ft³ ≈ 2,402.8 kg/m³. Going the other way, divide kg/m³ by 16.01846 — or remember water: 1,000 kg/m³ ≈ 62.428 lb/ft³.
Is g/cm³ the same as g/mL?
Yes, exactly — since 1964 the milliliter is defined as exactly one cubic centimeter, so both units are 1,000 kg/m³ and the converter shows identical numbers for them. (Between 1901 and 1964 the liter was tied to a kilogram of water and differed by 28 ppm; only historical high-precision chemistry data cares.)
What is a slug per cubic foot and who uses it?
The slug is the US customary mass unit that makes F = ma work without conversion factors (1 slug = 1 lbf·s²/ft = 14.5939 kg), so slug/ft³ is the density US fluid dynamics wants in Bernoulli and drag equations. Water is about 1.94 slug/ft³. Divide lb/ft³ by g = 32.174 ft/s² to get slug/ft³ — or convert through SI here and skip the gravity constant.
Which gallon does lb/gal use?
The US liquid gallon, 231 in³ exactly, giving 1 lb/US gal = 119.8264 kg/m³ — the unit drilling-mud weights and coating densities are quoted in (water ≈ 8.35 ppg, a 12 ppg mud ≈ 1,438 kg/m³). The imperial gallon is 4.54609 L, about 20% bigger, so an imperial lb/gal is a different number; convert imperial data explicitly before comparing.
Is density the same as specific gravity?
Specific gravity is density divided by a reference density, almost always water near 1,000 kg/m³ — which makes it numerically equal to g/cm³: steel at 7.85 g/cm³ has SG 7.85. It is dimensionless, so it never needs converting; multiply SG by 1,000 for kg/m³ or by 62.428 for lb/ft³, or just read both off the converter's all-units table.