Under API RP 13C, a shaker screen is rated by its D100 cut point — the particle size above which nothing passes — and labelled with an API screen number, not a mesh count. The API number equals the standard ASTM test sieve with the closest D100. Two screens carrying the same API number cut the same in the lab, whatever their “mesh.”
Why “mesh” is obsolete
Mesh counts the wires per inch — it never described the actual opening. Once layered and oblong-weave screens arrived, two panels could both be called “200 mesh” and pass completely different particle sizes. API RP 13C (internationally ISO 13501) fixed this by testing the real separation: a graded aluminium-oxide sample is sieved through the screen on a Ro-Tap against a stack of calibrated ASTM sieves, three runs averaged, to find the D100 cut point. The screen is then labelled with the API number of the closest ASTM sieve.
The chart: API number to D100 cut point
The API number is the ASTM sieve nearest the measured D100. Higher number = finer cut = smaller micron.
| API screen no. | D100 cut point (µm) | Typical role |
|---|---|---|
| API 20 | ~850 | Very coarse / scalping |
| API 30 | ~600 | Coarse, top-hole |
| API 40 | ~425 | Coarse |
| API 50 | ~300 | Surface / large cuttings |
| API 60 | ~250 | Intermediate |
| API 70 | ~212 | Intermediate |
| API 80 | ~180 | Intermediate |
| API 100 | ~150 (137.5–165) | Mid-section workhorse |
| API 120 | ~125 (116.5–137.5) | Mid-to-fine |
| API 140 | ~106 | Fine |
| API 170 | ~90 | Fine |
| API 200 | ~75 | Fine, late section |
| API 230 | ~63 | Very fine |
| API 270 | ~53 | Very fine |
| API 325 | ~45 | Finest practical |
| API 400 | ~38 | Finest practical |
What the API number does NOT tell you
The API number is a lab cut point, not a field-performance promise. How a screen actually behaves on the rig depends on the shaker design, motion and G-force, mud properties, ROP and bit type. For capacity, look at the second number on the label: conductance, in kilodarcies per millimetre (kD/mm) — the higher it is, the more flow the screen can pass. The third figure, non-blanked area, is the open area available to fluid.
Reading a screen label
A compliant RP 13C label gives three things: the API number (cut point), the conductance in kD/mm (throughput), and the non-blanked area (open area). Match cut point to the solids you need to remove, then use conductance to confirm the screen will carry your flow. Order by API number, never by “mesh.”
Key takeaways
The API number is the D100 cut point referenced to the closest ASTM sieve — a real, comparable number that mesh never gave you. Higher number, finer cut, smaller micron. Two screens at the same API number cut alike regardless of mesh. But the number rates separation, not field throughput — pair it with conductance, and let the shaker, mud and ROP decide which API number the section can actually run.