How to Choose Well Screen Slot Size and Gravel Pack: A Step-by-Step Guide with a Worked Example

Choose water well screen slot size from a sieve analysis: D10, D30, D60, uniformity coefficient, pack-aquifer ratio, slot chart and a worked example.

By Serkan İnkaya, Mert Aytünür, Ümit Can Özer, ADEN Wedge Wire · 19 October 2026 · 13 min read

Well screen slot size and gravel pack: the short answer

Well screen slot size in a water well is chosen from a sieve analysis of the aquifer sand, not from a catalogue. In a naturally developed well the slot retains approx. 40-50 % of the aquifer sand. In a gravel-packed well the slot retains 90 % of the gravel pack, and the pack is sized from the aquifer D30.

This guide goes through it the way we do it when a sieve analysis arrives with an inquiry: read the curve, decide natural development or gravel pack, size the pack, pick the slot, then check against a worked example. Slot sizes in the inquiries we receive run from 0.2 mm up to 2.75 mm, and some of them only say "slot 20", so we start with the slot numbers.

What does "slot 20" mean? Slot number to millimetres

The slot number is the slot width in thousandths of an inch. A "20 slot" screen has a 0.020 inch opening, which is 0.51 mm. In Europe and most export projects the slot is given in millimetres, so it helps to have both.

On a continuous-slot wedge wire screen (often called a Johnson-type screen) the slot is the gap between two turns of the V-shaped wire, measured at the outer face. Because the V-shaped wire narrows inward, the slot widens inward, so a grain that passes the outer face does not get stuck inside the slot.

How to read a sieve analysis for a water well

A sieve analysis gives the percentage of the sample that passes each sieve. Plotted on a log grain size axis, it gives the grading curve. Three points on this curve do most of the work:

• D10: the grain size that 10 % of the sample passes.
• D30: the grain size that 30 % passes.
• D60: the grain size that 60 % passes.

The uniformity coefficient is Cu = D60 / D10. A small Cu means the grains are close in size (uniform sand); a large Cu means a wide mix of sizes.

Water well literature uses "percent passing" as above. Oil and gas sand control uses the Coberly convention, where D10 is the size at which 10 % of the sample is coarser, so it is read from the other end of the curve. A "2 x D10" slot rule from an oil field paper means something quite different from a water well D10. If you are working on a production well rather than a water well, use our sand control method selection article instead.

Take samples from every aquifer layer that will be screened. The design is made on the finest material that will sit against the screen, not on an average sample.

Natural development or gravel pack?

The first decision is whether the aquifer can be developed naturally (fines pumped out, the coarser native sand left as a filter around the screen) or needs an artificial gravel pack. Rules from the NRCS National Engineering Handbook and common screen manufacturer practice:

A pack also lets you use a larger slot than the aquifer alone would allow, and a larger slot gives more open area on the same screen.

Slot size for a naturally developed well

For a well without an artificial pack, the slot is chosen so that the finer part of the aquifer passes during development and the coarser part bridges outside the screen. The published rules agree closely:

"Retains 40 %" and "passes 60 %" are the same point on the curve. With a stainless steel screen corrosion of the slot is less of a concern, but where encrustation is expected we lean to the conservative side.

How to design the gravel pack

The gravel pack is sized with the pack-aquifer ratio (P-A ratio): the D30 of the pack divided by the D30 of the aquifer. The NEH procedure, step by step:

1. Plot the sieve curve of the finest aquifer layer to be screened.
2. Multiply its D30 by the P-A ratio: x 4 for fine, uniform sand (Cu below 3); x 6 for coarse, non-uniform sand; up to x 9 for very non-uniform or silty material. This gives the D30 of the pack.
3. Draw a pack curve through this point with a Cu of approx. 2.5 or less.
4. Draw a +/- 8 % envelope around the curve as the permissible limits of the pack grading.
5. Select the slot so that it retains 90 % of the pack.
6. Specify washed, rounded, hard and insoluble grains.

NEH puts the lower limit of the ratio at 4 (below this the pack causes more head loss) and the upper limit at 9 (above this the pack becomes unstable). Other references give somewhat different bands: 3 to 8 in the Water Well Journal guidance, 4 to 6 in one US screen manufacturer's handbook. The Water Well Journal author also warns that a ratio above 8 risks a sand pumping well. When in doubt we recommend staying inside 4 to 6.

The pack material matters as much as its grading and has to meet these requirements:

Thickness is set by the annulus between borehole and screen. A pack thicker than approx. 125 mm (5 in) makes development harder. During placement the pack is fed through a tremie pipe; bridging usually comes from pouring too fast, too little water circulation, gravel too coarse for the annulus or obstructions such as centralisers.

Slot size for a gravel-packed well

Once the pack grading is fixed, the slot is chosen to retain 90 % of the pack, i.e. the slot is about the D10 of the pack material. For glass bead packs the slot retains 100 %. One US manufacturer notes that in a properly designed gravel-packed well a slot below 1.0 mm is rarely needed; this is a manufacturer view, and the 90 % rule applied to the actual pack grading still decides.

A slot taken from the aquifer curve in a packed well would be too small and would cost open area for no gain in sand control.

Worked example

Assumed input: a fine, uniform sand aquifer, D30 = 0.35 mm, Cu = 2.2. The aim is a gravel-packed well with a stainless steel continuous-slot screen.

The next step is open area and screen length. As an example from the same calculation set, a DN 250 screen (OD 273 mm) with a 3.0 mm wide wire and a 1.0 mm slot has 25 % open area: open area = slot x 100 / (slot + wire width). At the usual 3 cm/s entrance velocity this gives approx. 6.4 l/s per metre of screen. These are gross figures; slots touching the gravel pack are always partly blocked by the grains, so they are a basis for comparison and not a guaranteed yield. Diameter, open area and entrance velocity are covered in our separate article, and you can try your own numbers in the open area calculator.

Common mistakes we see in inquiries

• Designing on one sample. A single composite sample hides the finest layer, and the finest layer decides the slot.
• Choosing the slot from a general rule such as "slot = 80-90 % of the particle to be retained". That rule from industrial screening does not work for wells; use the sieve curve rules above.
• Packs that are too coarse. A P-A ratio above 8 to 9 lets aquifer sand move into the pack, and the well pumps sand.
• Mixing up the D10 conventions of water wells and oil wells.
• Matching the slot to the aquifer in a gravel-packed well, which gives a slot too small for the pack.
• Specifying only "slot 20" without the sieve data. We can supply the slot, but we cannot check whether it suits the formation.

In German-speaking markets, tenders specify wire-wrapped well screens under DIN 4900-2:2017; the older DIN 4935 was withdrawn in 2017 and should not be referenced in new documents.

Send us your sieve analysis

ADEN Wedge Wire is a wedge wire screen manufacturer based in Istanbul, Turkey, with a European office in Sindelfingen, Germany. Send your sieve analysis or borehole log, the screen diameter and the depth. We reply within 24 hours and follow up with a proposal for slot size, gravel pack grading and screen open area, free of charge. A drawing or tender specification is also fine as a starting point. Contact: info@adenwedgewire.com or the RFQ form.

Johnson Screens is a trademark of its owner. ADEN Wedge Wire is not affiliated with Johnson Screens. The term "Johnson-type" is used here only to describe a screen design.

Frequently asked questions

References

1. National Engineering Handbook, Part 631, Chapter 32, Well Design and Spring Development, USDA Natural Resources Conservation Service
2. Selection of Filter Pack and Slot Size (2024), Water Well Journal
3. DIN 4900-2:2017, Brunnenfilter aus nichtrostendem Stahl, DIN