Coroplast thickness is measured in millimetres and it buys stiffness, not waterproofing. All corrugated plastic is equally waterproof. 4mm is the standard for campaign and open-house signs, 6mm and 8mm hold their plane through a full season, and 10mm is for large exposed panels that would otherwise bow.
A yard sign is the cheapest square footage of advertising you can buy, but only if it stays up, stays flat, and can actually be read from a moving car. Those three outcomes are decided by choices most people click straight through. Here is what each one does.
Thickness, size and real cost, side by side
Every price below is for a single sign, printed one side, standard turnaround, at the time of writing. We publish these because most print sites will not, and comparing material grades without prices is not much use to anyone.
| Material | Thickness | 6 x 24 | 12 x 18 | 18 x 24 | vs 4mm |
|---|---|---|---|---|---|
| 4mm coroplast | 4mm / 0.157" | $16.48 | $11.80 | $17.10 | baseline |
| 4mm foam board | 4mm / 0.157" | $18.58 | $20.88 | $25.70 | 1.5x |
| 6mm coroplast | 6mm / 0.236" | $20.92 | $24.95 | $31.58 | 1.85x |
| 8mm coroplast | 8mm / 0.315" | $22.15 | $27.40 | $35.25 | 2.06x |
| 10mm coroplast | 10mm / 0.394" | $24.80 | $27.98 | $37.00 | 2.16x |
Prices as of September 2026, single sign, one side, standard turnaround. Check the product page for current pricing and volume rates, which fall substantially per sign as quantity rises.
Two things worth noticing in that table.
First, 12 x 18 in 4mm is the best value in the whole range. At $11.80 it is cheaper than the smaller 6 x 24 rider size, because 12 x 18 is the volume size the industry gangs up and runs constantly. If your message fits, that is the size to pick.
Second, going from 4mm to 10mm roughly doubles the cost. That is a real decision, not a small upgrade, and for most weekend and campaign use it is not worth it.
Coroplast or foam board?
These get confused constantly, and they are not interchangeable.
Coroplast is corrugated plastic. Hollow vertical flutes run through it like miniature cardboard channels. It is waterproof, it does not absorb water at all, and those flutes are what let an H-stake slide up inside the sign. It lives outdoors.
Foam board is a foam core sandwiched between two paper or plastic facing sheets. It is lighter, flatter, and takes ink beautifully, which makes it the better looking of the two indoors. But the core is foam and the facing is usually paper. Get it properly wet and the facing delaminates and the board warps. It does not recover.
So the rule is simple. Outdoors, staked in ground, exposed to weather: coroplast. Indoors, on an easel, at a trade show or a reception desk: foam board. A foam board sign in a Florida summer is a one-week sign.
Note from the table that foam board is also the more expensive of the two 4mm options at every size. You are paying for appearance, not durability.
There is a lot more to this one, including why the delamination is permanent and where foam board is genuinely the better choice. Full comparison here: coroplast vs foam board.
Flute direction, and what "H-Stand Compatible" actually means
This is the option most people do not understand, including plenty of people who sell signs.
Coroplast flutes run in one direction only. An H-stake works by sliding its two vertical legs up inside those hollow flutes, which is why a coroplast sign needs no drilled holes to mount. For that to work, the flutes have to run vertically relative to how the sign will stand.
On a portrait sign the flutes usually already run the right way. On a landscape sign such as 24 x 18, the sheet may have to be cut with the flutes running the short dimension instead, which is a different production orientation. That is what the H-Stand Compatible option controls.
Get it wrong and the stake legs meet a closed wall of plastic and simply will not go in. You then end up drilling the sign, which defeats the point and weakens it.
The short version: if the sign is going on an H-stake, say so when you order. If it is going on a fence, a wall or a frame, it does not matter.
Grommets or stakes, pick your mounting first
Grommets are reinforced metal rings for hanging or zip-tying a sign. They are not a default, they are a mounting choice, and the wrong choice means holes you did not want.
- Going on an H-stake in the ground? No grommets. The stake legs slide up inside the flutes, so no holes are needed at all.
- Zip-tying to a chain-link fence? Corner grommets.
- Hanging in a frame or on a real-estate post? The frame's pattern, usually top corners or a top edge set.
- Large exposed panel? Grommets every two to three feet, so wind load spreads across many fixings instead of tearing out one.
One stake or two?
One H-stake per sign up to 18 x 24 in normal soil. Two stakes for anything 24 inches wide or wider, because a single stake in a wide sign lets it pivot in wind until the flutes tear around the legs.
Push stakes in by the horizontal bar, never by pressing on the sign itself, and get the legs at least six inches down. In dry or sandy ground, go deeper. In Florida sand, expect to go deeper still or add a second stake earlier than the size rule suggests.
Sizing for reading distance
Allow roughly ten feet of readable distance for every inch of letter height. That is the rule the sign trade works to and it holds up well in practice.
| Letter height | Readable from | Suits |
|---|---|---|
| 1" | ~10 ft | Walk-up only. Reception desk, table sign |
| 2" | ~20 ft | Pedestrian traffic, driveway |
| 3" | ~30 ft | Residential street, open house |
| 4" | ~40 ft | Slow traffic, parking lots |
| 6" | ~60 ft | 35 to 40 mph road |
| 8" | ~80 ft | Highway frontage |
An 18 x 24 sign with a 3 inch headline reads from about 30 feet, which is fine for a residential street. For traffic moving at 40 mph, go to 24 x 36 or larger, cut the message to six words, and make the phone number the second biggest element on the sign. High contrast beats clever design at every speed.
Single or double sided?
- One side for signs against a wall or fence, or where traffic only passes one way.
- Two sides, same image for signs staked where traffic passes both directions. On our 4mm signs this is a small uplift, around $1 to $2 per sign at low quantities.
- Two sides, different images when each direction needs its own message. Opposite-pointing arrows are the classic case.
Common questions
What thickness coroplast is best for yard signs?
4mm for campaign signs, open houses, event directionals and anything up to 18 x 24 that comes down within a few months. 6mm or 8mm if the sign stays out for a full season or spans a wide panel. 10mm only for large exposed signage.
Is thicker coroplast more waterproof?
No. All corrugated plastic is equally waterproof regardless of thickness. Thickness buys stiffness and resistance to bowing, nothing else.
How long does a coroplast yard sign last outside?
The plastic itself lasts years. What fails first is the ink under UV. Expect good colour for a season of direct Florida sun and noticeable fading beyond that. Signs in shade last considerably longer.
Can you use foam board outside?
Not for long. The foam core is fine but the facing sheets are usually paper, and once they get wet they delaminate and the board warps permanently. For anything outdoors use coroplast.
What does 4mm coroplast mean?
The sheet is 4 millimetres thick, which is about 0.157 inches. It is the industry standard weight for yard signs and the most widely stocked grade.
Do I need grommets for a yard sign?
Not if it is going on an H-stake. The stake legs slide up inside the hollow flutes, so no holes are needed. Order grommets only if you are hanging, zip-tying or frame-mounting the sign.
How many H-stakes per sign?
One up to 18 x 24 in normal soil. Two for anything 24 inches wide or wider, and two earlier than that in sandy ground.
What size yard sign should I order?
12 x 18 if the message is short, because it is the cheapest size we print and the one the industry runs in volume. 18 x 24 is the standard for real estate and open houses. Go to 24 x 36 or larger only when traffic is moving faster than about 35 mph.
Before you order
On hard surfaces where stakes cannot go in, plan on a fence fixing or a wall mount and order the matching grommet pattern rather than trying to solve it on site.
And as with everything we print, artwork gets a free hand check before production. Bleed, resolution, colour mode and readable type size, looked at by a person. On signs the most common thing we catch is type that will be unreadable at the distance the sign is actually going to be viewed from.