What Does 4mm Coroplast Actually Mean? Flute Direction, Thickness and Wind
4mm coroplast is a corrugated polypropylene sheet that measures 4 millimetres, or 0.157 inches, from face to face. It is built like miniature cardboard: two flat plastic skins separated by internal ribs called flutes. The 4mm figure is the total panel thickness, not the thickness of either skin. The single most important thing to understand about it is flute direction, because the flutes run one way only, and that direction decides which edge of the sign slides onto a step stake and which way the panel folds when the wind hits it.
Key takeaways
- 4mm equals 0.157 inches, roughly 5/32 of an inch, and it is the standard thickness for yard signs and real estate signs.
- Coroplast is polypropylene, not cardboard and not PVC, so it does not absorb water and it will not delaminate in rain.
- Flutes are the hollow channels inside the sheet. They give the panel almost all of its stiffness along one axis and almost none across the other.
- A step stake only works when the flutes run parallel to the stake legs, which for a standard 18×24 portrait yard sign means vertical flutes.
- At Get Quick Signs a 48×96 sheet of single sided 4mm coroplast is 66.00 dollars, and an 18×24 sign yields 8 up per sheet, so the material works out to 8.25 dollars per sign.
- If the sign is going somewhere genuinely windy, 10mm coroplast is the upgrade, not thicker ink or more grommets.
How thick is 4mm coroplast, exactly?
4mm is 0.157 inches. Suppliers list the sheet as 0.157 x 48 x 96, and U.S. Plastic Corp catalogues the 4mm 48 in x 96 in white Coroplast sheet at that exact figure. That is thinner than most people picture. It is about the thickness of two stacked credit cards, and you can flex a full sheet with one hand.
The thickness is deceptive because coroplast gets its rigidity from geometry rather than mass. Professional Plastics describes Coroplast as a twinwall polypropylene sheet, meaning the two skins are held apart by the flute walls in the same way an I beam holds its flanges apart. Along the flutes the panel is stiff. Across them it hinges.
What are flutes and why does flute direction matter?
Flutes are the parallel hollow channels running through the middle of the sheet. Hold a piece of coroplast up to a light and you will see the stripes. On a 48×96 sheet they normally run the 96 inch direction, though sheets are stocked both ways, which is why blank suppliers list flute length as a separate spec line, for example an 18 x 24 blank with 24 inch flutes.
Direction matters for three practical reasons:
- Stakes. The two legs of a step stake push up inside two flutes. If the flutes run across the sign instead of up and down, there is nothing for the stake to enter.
- Stiffness. A sign bends easily across the flutes. A 24×18 landscape sign with vertical flutes stays flat. The same sign with horizontal flutes sags in the middle.
- Appearance. Flute lines are faintly visible through the print, especially on large flat colour areas. On a portrait sign vertical lines read as intentional. Horizontal lines read as a defect.
When you configure a sign on our coroplast sign builder, the standard sizes assume the flutes run the way a step stake needs them. For 48×48 we list vertical flutes and horizontal flutes as separate layouts because at that size neither direction is obviously correct.
4mm or 10mm coroplast: which should I order?
| Spec | 4mm coroplast | 10mm coroplast |
|---|---|---|
| Thickness | 0.157 in | 0.394 in |
| Material | Twinwall polypropylene | Twinwall polypropylene |
| Sheet price, single sided | 66.00 dollars | 105.00 dollars |
| Sheet price, double sided | 82.50 dollars | 135.00 dollars |
| 18×24 signs per 48×96 sheet | 8 | 8 |
| Material cost per 18×24 sign | 8.25 dollars | 13.13 dollars |
| Takes a standard step stake | Yes | Yes, with a looser fit |
| Best for | Campaign, real estate, event, directional | Exposed sites, long placements, larger panels |
Per sheet pricing drops at volume. From 10 sheets, single sided 4mm falls to 49.50 dollars per sheet, and from 51 sheets it falls to 45.00 dollars. On an 18×24 that is 6.19 dollars and 5.63 dollars of material per sign respectively.
Does 4mm coroplast actually hold up in wind?
For a normal yard placement, yes. The failure mode is almost never the panel tearing. It is the stake legs elongating the flutes until the sign works itself loose, or the sign folding along a flute line because the wind is hitting it side on. Suppliers make the same point plainly: SignWarehouse recommends stepping up to 10mm corrugated plastic for higher wind areas or longer term outdoor installs rather than trying to make 4mm survive a site it was not chosen for.
Three things that genuinely help: orient the flutes vertically so the stiff axis fights the gust, use a heavy duty stake in soft ground, and keep the sign below 24×36 in exposed locations. Adding grommets to a coroplast sign is useful when you are zip tying to a fence, not when you are staking into turf. Grommets on coroplast are 0.38 dollars per sign plus a 22.50 dollar one time setup.
What does a 4mm coroplast sign cost?
Material is the base, then the finishing adds up. A 25 piece run of 18×24 single sided signs needs 4 sheets at 66.00 dollars, so 264.00 dollars of substrate, plus 25 standard step stakes at 1.88 dollars each, so 47.00 dollars. We break the full arithmetic down in the 2026 sign price index, and the practical setup guidance lives in our yard sign and step stake guide. Stakes are also sold on their own as a 50 count box or a heavy duty 25 count box.
Frequently asked questions
Is 4mm coroplast waterproof?
Yes. It is polypropylene, so it does not absorb water. Water can sit inside the open flutes at a cut edge, which is cosmetic rather than structural, and it drains once the sign is stood upright.
Can you print 4mm coroplast on both sides?
Yes. Double sided printing on 4mm raises the sheet price from 66.00 dollars to 82.50 dollars, an increase of 25 percent, not 100 percent, because the sheet and the handling are already paid for.
How long does a coroplast yard sign last outdoors?
Plan on one season to about a year of continuous exposure. Polypropylene chalks and gets brittle under UV before the print fails. For multi year outdoor use, choose aluminum or PVC instead.
Can I cut coroplast myself?
Yes, with a sharp utility knife against a straight edge. Cut with the flutes where you can. Cutting across the flutes leaves an open honeycomb edge that looks rough.
What is the largest coroplast sign you can make?
One panel is limited by the 48×96 sheet. We list sizes up to 48×96 and 36×96 as single piece signs on the builder.
Build your coroplast signs
Pick a size, choose single or double sided, add stakes and see live per sign pricing on the coroplast sign builder. If you are still deciding between substrates, browse the full rigid signs range or start at the shop.
