Definitions
| Name | Symbol | Value | Unicode |
|---|---|---|---|
| Percent | % | 1/100 = 10⁻² | U+0025 |
| Per mille (per thousand) | ‰ | 1/1,000 = 10⁻³ | U+2030 |
| Per ten thousand (basis point) | ‱ | 1/10,000 = 10⁻⁴ | U+2031 |
| Parts per million | ppm | 10⁻⁶ | — |
| Parts per billion | ppb | 10⁻⁹ (short-scale billion) | — |
| Parts per trillion | ppt | 10⁻¹² (short-scale trillion) | — |
Conversion chart
To convert, multiply by the ratio of the two units' powers of ten. Moving one row down multiplies the number by 10, 1,000 or more.
| Fraction | Percent | Per mille | ppm | ppb |
|---|---|---|---|---|
| 1 | 100% | 1,000‰ | 1,000,000 | 1,000,000,000 |
| 0.1 | 10% | 100‰ | 100,000 | 100,000,000 |
| 0.01 | 1% | 10‰ | 10,000 | 10,000,000 |
| 0.001 | 0.1% | 1‰ | 1,000 | 1,000,000 |
| 0.0001 | 0.01% | 0.1‰ | 100 | 100,000 |
| 1 × 10⁻⁶ | 1 × 10⁻⁴% | 0.001‰ | 1 | 1,000 |
| 1 × 10⁻⁹ | 1 × 10⁻⁷% | 1 × 10⁻⁶‰ | 0.001 | 1 |
Worked examples and usage notes
- 250 ppm = 250 × 10⁻⁶ = 0.025% = 0.25‰
- 3.5% = 35‰ = 35,000 ppm
- A 0.25 percentage-point change in an interest rate is 25 basis points (25‱)
- “ppt” is ambiguous: it can mean parts per trillion (10⁻¹²) or, in oceanography and some older texts, parts per thousand (10⁻³). State the power of ten when it matters
- NIST and ISO treat % as acceptable with the SI but advise against ppm, ppb and ppt because they are language-dependent; write 2 µg/g or 2 × 10⁻⁶ instead
- Always say what the ratio is of: mass fraction (mg/kg), volume fraction (µL/L) or amount fraction (µmol/mol) can differ for the same sample
Sources: NIST: NIST Guide to the SI (SP 811), Chapter 7: Rules and Style Conventions