Well Screen Open Area, Diameter and Entrance Velocity: How to Size a Screen for Your Yield

Size a water well screen: open area formula and table, capacity per metre at 3 cm/s, the 0.1 ft/s debate, diameter from pump and upflow, worked example.

By Serkan İnkaya, Mert Aytünür, Ümit Can Özer, ADEN Wedge Wire · 15 October 2026 · 17 min read

Well screen open area, diameter and entrance velocity: the short answer

On a continuous-slot wedge wire screen (often called a Johnson-type screen), open area % = slot x 100 / (slot + wire top width). With the outside diameter it gives the capacity per metre of screen at an entrance velocity of usually 3 cm/s (0.1 ft/s). Pump size and upflow velocity set the diameter; a bigger bore adds little yield.

Open area and capacity per metre are asked in nearly every well screen inquiry we get, and quite often the tender writes 3 cm/s as a hard acceptance limit. Below we go through how we size a screen for a quotation and where the 3 cm/s rule comes from. The worked example at the end can be repeated in our flow rate calculator.

How do you calculate well screen open area?

On a wire-wrapped screen the open area depends only on two numbers: the slot s and the top width w of the V-shaped wire. The formula is open area % = s x 100 / (s + w). Diameter does not enter. A 1.0 mm slot with a 2.8 mm wide wire gives 1.0 x 100 / 3.8 = 26.3 %, whether the screen is 4 in or 16 in.

The table gives gross open area for our standard V-wire profiles. The wire top widths are: AW28 2.2 mm, AW34 2.8 mm, AW42 3.4 mm, AW50 5.0 mm.

A larger slot raises open area quickly, which is one reason a gravel pack is useful: it allows a larger slot than the aquifer alone would (see our article on slot size and gravel pack). A wider wire lowers open area. Wider and taller wire is chosen because the screen goes deeper and needs more collapse resistance, so in a deep well some open area is always traded for strength. Which profile a well needs is a project calculation for the installation depth and the placement method; background on the profiles is in V-wire profile performance.

You can try your own slot and wire combination in the open area calculator.

Gross and effective open area

Every open area figure in a catalogue, ours included, is a gross value. One European screen catalogue writes that slots in contact with the gravel pack are always partly blocked by the grains. The support rods also cover part of the slot length on the inside. So the effective inlet area in a finished well is lower than the table, by an amount nobody can state in advance.

We therefore use open area and transmitting capacity tables to compare screens on the same basis; they are no guarantee of yield. The yield of a well is set by the aquifer, the well design and the development. NEH adds a remark to its own example calculation: these calculations "are an analysis of the hydraulic characteristics of the screen itself at an arbitrary entrance velocity and do not consider the hydraulic characteristics of the aquifer."

If a bid compares two screens, check that both open area figures were computed with the same formula and on the wire width actually offered.

Transmitting capacity per metre

Transmitting capacity is the flow one metre of screen passes at the design entrance velocity. In US units NEH gives it as gpm per foot = open area (in² per ft) x 0.31, where 0.31 is 0.1 ft/s converted to gallons per minute per square inch of open area. In metric units the same thing is:

Q (l/s per m) = pi x OD (m) x open area (fraction) x v (m/s) x 1000

For a DN 250 screen (OD 273 mm), slot 1.0 mm, AW42 wire (open area 22.7 %) and v = 0.03 m/s:

Q = pi x 0.273 x 0.2273 x 0.03 x 1000 = 5.85 l/s per metre (approx. 21 m³/h per metre). The open area is used unrounded here (1.0 / 4.4 = 0.2273); with the rounded 0.227 you get 5.84.

With AW34 wire (26.3 %) the same screen gives 6.77 l/s per metre, and with a 3.0 mm wire (25 %) 6.43 l/s per metre. The flow rate calculator uses the same formula; its water well preset is 0.03 m/s, and it lets you deduct the rod coverage if you know the rod width and spacing.

Is the 0.1 ft/s (3 cm/s) entrance velocity rule still valid?

Entrance velocity is the mean speed of water through the slot openings, i.e. flow divided by open area. The traditional limit is 0.1 ft/s, which is 0.0305 m/s, rounded to 3 cm/s in Europe. NEH describes it as a criterion that "approaches but does not exceed 0.1 foot per second" and says it is in most published water well standards. The same chapter then notes that AWWA A100-06 no longer stipulates a maximum screen entrance velocity, because research and testing showed that the allowable velocity depends on the aquifer, the overall well design, the intended performance and the water quality.

The published positions sit quite far apart:

NEH links the velocity limit to the amount of sand carried into the well, the head loss in the screen and the rate of incrustation or corrosion. The counter view questions both ends: it reports minimal head loss up to approx. 1 m/s and sees no proven link to corrosion or incrustation. In German-speaking markets the wire-wrapped well screen itself is specified under DIN 4900-2:2017 (the older DIN 4935 was withdrawn in 2017); the 3 cm/s figure comes from design practice and catalogues.

In our quotations we calculate at 3 cm/s, since both the US and the European traditions use it and it puts every bidder on the same basis. We do state that it is a design convention. If a tender writes 3 cm/s as an acceptance limit with no tolerance, bids become hard to compare, because screen length and open area can be traded against each other.

How to choose well screen diameter

Diameter is chosen in two steps: first the pump has to fit in the casing, then the upward velocity inside the screen has to stay at or below approx. 1.5 m/s. A screen telescoped below the pump does not need the pump diameter, but it still has to carry the full well yield upwards.

For the pump, the Washington State well construction rules (WAC 173-160-211, Table 3, in line with the classic US well handbook table) give the following. We added the l/s column.

For a turbine pump NEH asks for casing at least 2 in larger than the pump bowls down to the lowest expected pump setting.

For the screen, the upflow limit gives a maximum flow per inside diameter, Q max = 1.5 x pi x ID x ID / 4:

The real ID of a wire-wrapped screen depends on the wire height and the rods, so check it on the data sheet before you rely on the last 5 %.

Does a bigger diameter give more water?

Only a little. NEH Table 32-4 compares wells of the same depth in the same formation. Going from 6 in to 12 in raises yield by about 10 %, from 12 in to 24 in by about 13 %. From 6 in the gains are:

NEH's advice: to increase yield, increase the screen length, not the diameter; doubling the length nearly doubles the yield. A bigger diameter is still needed where the upflow limit or the entrance velocity calls for it.

How long should the screen be?

Screen length comes from the aquifer first and from the capacity check second. NEH gives these rules:

• Confined (artesian) aquifer: screen the full thickness of the aquifer, and keep the maximum drawdown above the top of the screen.
• Unconfined aquifer: screen the lower third of the aquifer and keep drawdown above the top of the screen. About 85 % of the possible yield is reached at about 60 % drawdown.
• When the screen is telescoped through the casing, add at least 5 ft (1.5 m) of blank pipe of the screen diameter on top, reaching into the casing, and seal the gap.
• For small wells, a short screen in a coarse layer is better than a long screen in a fine one.

One US screen manufacturer also warns that screening only chosen intervals, with blank pipe between them, can shut off productive parts of the aquifer. If the borehole log shows several good layers, screen them all.

Then divide the design flow by the capacity per metre. If the aquifer rule gives a longer screen than the capacity check, the entrance velocity simply ends up lower. If the capacity check asks for more screen than the aquifer can take, change the slot, the wire or the diameter, in that order.

Worked example: 50 l/s from an unconfined aquifer

Assumed input: target yield 50 l/s, unconfined sand aquifer with 30 m saturated thickness, gravel pack designed as in our slot size article (slot 1.0 mm). The heavier AW42 wire is assumed for the installation depth; the final profile is checked per project.

With AW34 wire the same screen would need 50 / 6.77 = 7.4 m, so the aquifer rule decides in both cases. Remember that 6 m pieces do not fit a 20 ft container (internal length 5.898 m); crated pieces should be approx. 5.7 m net or less, otherwise the screen goes in a 40 ft container.

The same check in US units is in NEH: 400 gpm through 10 ft of 0.040 in slot screen. An 8 in screen with 87 in² per ft of open area gives 400 x 0.32 / 870 = 0.15 ft/s, too high. A 12 in screen with 130 in² per ft gives 0.099 ft/s, which passes.

Common mistakes we see in inquiries

• Open area quoted for a thin wire and collapse strength for a thick one. Both have to belong to the same profile.
• Capacity tables used as a guaranteed yield, although they are gross values for comparing screens.
• A larger diameter chosen to get more water, when more screen length or a larger slot would do more.
• Upflow inside a small screen never checked at high yield.
• Screen length set by the piece length that fits the truck instead of by the aquifer.

Send us your target yield and depth

ADEN Wedge Wire is a wedge wire screen manufacturer based in Istanbul, Turkey, with a European office in Sindelfingen, Germany. Send the target yield, the installation depth and the borehole log or sieve analysis, and we reply within 24 hours. We then follow up with the sizing (diameter, slot and screen length) and the calculation sheet together with the quotation. 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 (2010), USDA NRCS
2. Washington Administrative Code WAC 173-160-211, Table 3, Washington State Legislature
3. DIN 4900-2:2017, Brunnenfilter aus nichtrostendem Stahl, DIN