Enter the percentage of the major enantiomer and the percentage of the minor enantiomer into the calculator to determine the enantiomeric excess.
Enantiomeric Excess Formula
The calculator uses one of three formulas depending on the mode you select.
From amounts or percentages:
ee (%) = |A − B| / (A + B) × 100
From a known ee (composition):
major (%) = (100 + ee) / 2 minor (%) = (100 − ee) / 2
From optical rotation:
ee (%) = (observed rotation / pure enantiomer rotation) × 100
- A, B — measured amount of each enantiomer (mol, mass, HPLC peak area, or %)
- ee — enantiomeric excess, expressed as a percentage
- major / minor — percentage of each enantiomer in the mixture
- observed rotation — specific rotation measured for the sample
- pure enantiomer rotation — literature specific rotation of the pure enantiomer at matching solvent, concentration, temperature, and wavelength
Both A and B must be measured in the same units. The optical rotation method assumes the sample contains only the two enantiomers (no impurities, no other chiral species) and that conditions match the reference value. A racemic mixture has 0% ee; a single enantiomer has 100% ee.
Reference Tables
Use these tables to interpret your result and convert quickly between common ee values.
| % ee | Major : Minor | Major % | Minor % |
|---|---|---|---|
| 0 | 1 : 1 | 50 | 50 |
| 20 | 1.5 : 1 | 60 | 40 |
| 50 | 3 : 1 | 75 | 25 |
| 80 | 9 : 1 | 90 | 10 |
| 90 | 19 : 1 | 95 | 5 |
| 95 | 39 : 1 | 97.5 | 2.5 |
| 98 | 99 : 1 | 99 | 1 |
| 99 | 199 : 1 | 99.5 | 0.5 |
| % ee Range | Typical Interpretation |
|---|---|
| 0 – 5 | Racemic or near-racemic; no useful selectivity |
| 5 – 50 | Low to moderate; method needs optimization |
| 50 – 90 | Good; useful for many synthetic applications |
| 90 – 99 | High; common target for asymmetric catalysis |
| ≥ 99 | Near enantiopure; pharmaceutical-grade target |
Worked Example
An HPLC trace gives peak areas of 85 for the (R) enantiomer and 15 for the (S) enantiomer.
ee = |85 − 15| / (85 + 15) × 100 = 70 / 100 × 100 = 70% ee
Composition: major = (100 + 70) / 2 = 85%; minor = (100 − 70) / 2 = 15%. Ratio = 85 : 15, or about 5.67 : 1.
FAQ
Is ee the same as enantiomeric ratio (er)? No. ee is the absolute difference between the two enantiomers as a percentage of the total. er is the direct ratio, often written as 85:15. Convert with er = (100 + ee)/2 : (100 − ee)/2.
Why does my optical-rotation ee read above 100%? Either the reference rotation you used was measured under different conditions (solvent, concentration, temperature, wavelength) or the sample contains a chiral impurity that boosts the rotation. Recheck the reference and the sample purity.
Can I use HPLC peak areas directly? Yes, as long as both enantiomers have the same response factor at the detector wavelength, which is true for UV detection of enantiomers on a chiral column.
What if A equals B? The mixture is racemic and ee is 0%.
