The PD Journal
Near PD vs Distance PD: Calculator, Formula and Conversion Table
Last updated: October 2026 | Reading time: 7 minutes
Short answer: near PD is smaller than distance PD because your eyes turn inward to read. A common optical rule of thumb is near PD = distance PD × WD ÷ (WD + 27), where WD is your reading distance in millimetres. At 40 cm that works out to roughly 94% of your distance PD, a reduction of roughly 4 mm for typical adults.
Why they differ
When you look into the distance, your lines of sight are almost parallel. When you read, both eyes rotate toward the nose. The point where each line of sight crosses the lens moves inward, so the lens centres for reading glasses sit closer together than the lens centres for distance glasses.
The formula
Opticians use a simple estimate:
Near PD = Distance PD × WD ÷ (WD + 27)
- WD = working (reading) distance in mm, measured from the glasses.
- 27 = a fixed "stop distance" in mm, roughly the distance from the lens to the eye's centre of rotation plus the distance from the eye to the lens.
This is a practitioner rule of thumb, not a legal standard, and it ignores prescription effects. A discussion among opticians is archived on Optiboard. When in doubt, have an optician measure your near PD at your real reading distance.
Conversion table (calculated from the formula)
| Distance PD | 30 cm | 33 cm | 40 cm | 50 cm |
|---|---|---|---|---|
| 58 mm | 53.2 | 53.6 | 54.3 | 55.0 |
| 60 mm | 55.0 | 55.5 | 56.2 | 56.9 |
| 62 mm | 56.9 | 57.3 | 58.1 | 58.8 |
| 64 mm | 58.7 | 59.2 | 60.0 | 60.7 |
| 66 mm | 60.6 | 61.0 | 61.8 | 62.6 |
| 68 mm | 62.4 | 62.9 | 63.7 | 64.5 |
All values are in millimetres, rounded to 0.1.
Worked example
Distance PD 63 mm, reading at 40 cm (400 mm):
63 × 400 ÷ (400 + 27) = 63 × 0.937 = 59.0 mm
Which one do you need?
- Distance single-vision glasses: distance PD.
- Reading-only glasses: near PD at your reading distance (see PD for reading glasses).
- Computer glasses: near or intermediate PD at about 50 to 70 cm (see PD for computer glasses).
- Progressive lenses: distance PD plus fitting height, measured by an optician (see PD for progressive lenses).
How our tool handles it
If you choose near PD, the tool measures your distance PD first and then applies the near conversion for a 40 cm reading distance. If you choose distance PD and take the photo at arm's length, the tool also corrects for the fact that your eyes were turned slightly inward toward the camera.
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