The front of a caulk tube is an advertisement. The back is a contract. I read both, in that order, and I believe the back.
Once you know where to look, every label answers the same five questions: what is it made of (A), what does it claim to do (B), how long until it's ready (C), how is it rated and how much is in there (D), and is it still fresh (E). Let's take them one at a time.
A. What it's made of
This is the most important line on the tube, and it's often the smallest. The chemistry tells you nearly everything: whether it's paintable, how it handles water, how much it can flex, and how you'll clean it up. Our formulations guide covers each one in depth, but here's how to spot them.
- "100% silicone" means silicone. Waterproof, flexible, great in showers, and not paintable. See silicone.
- "Siliconized," "with silicone," "silicone plus," or "silicone-enhanced" almost always means acrylic latex with some silicone added. It's paintable and cleans up with water. It is not silicone, and in a shower it won't last like silicone. This is the single most common mix-up in the aisle.
- "Acrylic latex" or "painter's caulk" is the paintable interior workhorse. See acrylic latex.
- "Elastomeric" on a water-based tube usually means a stretchier acrylic built for exteriors. See elastomeric latex.
- "Polyurethane" or "urethane" is the tough, paintable exterior and concrete option. See polyurethane.
- "Hybrid," "MS polymer," "SMP," "silyl-terminated," or "advanced polymer" points to the moisture-curing hybrids. See MS polymer.
- "Butyl" is the sticky gutter, flashing, and roofing specialist. See butyl.
If the front only says something vague like "advanced sealant," turn it over. Cleanup instructions are a giveaway: "clean up with water" means water-based latex; "mineral spirits" usually means polyurethane, solvent-based, or butyl.
B. The claims
Most claims are true in a narrow sense. Here's the narrow sense.
- Mold- or mildew-resistant
- The caulk contains a mildewcide that discourages growth on the cured bead. It doesn't stop mold in soap film on top of the bead, or in a wet joint behind it. You still have to clean and ventilate. See why shower caulk turns black.
- Waterproof
- Once fully cured, in the conditions it's designed for. Not while it's wet, and not if the joint wasn't clean.
- Paintable
- You can paint it, usually after a set time. Some paints behave better than others over some caulks, so test if it matters.
- Stretches X%
- Often a lab elongation figure: how far a sample stretches before it breaks. What matters more for a real joint is movement capability, how much it can stretch and compress over and over. That's in the ASTM rating (D).
- Crack-proof, won't shrink
- Relative claims. Everything cracks in the wrong joint, and every water-based caulk shrinks somewhat as it dries.
- Lifetime, 35-year, 50-year
- A durability or warranty claim, with conditions. Real life depends on prep, joint design, and exposure far more than the number on the front.
- Low odor, low VOC
- Fewer volatile organic compounds, which is nicer indoors. See VOC.
- Interior/exterior
- Rated for both, but "rated for exterior" isn't the same as "best for exterior." Outdoors, check the movement class.
C. The times
Labels list several times, and they're easy to confuse. "Paintable in 30 minutes" is about when paint can go on. "Rain-ready" or "water-ready" is about when it won't wash out. "Shower-ready" is a specific claim some silicones make about when you can use the shower. And "full cure" is when it's reached its final strength, which is usually much later. The asterisk next to a fast time usually leads to fine print about temperature, humidity, and bead size. Our weather and cure guide explains why those conditions matter so much.
D. The rating and the size
ASTM C920, decoded
High-performance joint sealants are classified under ASTM C920, the Standard Specification for Elastomeric Joint Sealants. A full rating reads like a license plate, something like "Type S, Grade NS, Class 25, Use NT, M, G, A, O." Here's what each piece means.
| Part | Options | What it tells you |
|---|---|---|
| Type | S or M | Single-component (ready to use from the tube) or multi-component (mixed before use) |
| Grade | NS or P | Non-sag, which stays put in vertical joints, or pourable, which self-levels in horizontal joints |
| Class | 12.5, 25, 35, 50, 100/50 | Movement capability, as a percentage of joint width. Class 25 handles ±25%; Class 100/50 handles +100% and −50% |
| Use | T, NT, I | Traffic (walkable joints like pavement), non-traffic, and immersion (continuously underwater) |
| Substrates | M, G, A, O | Tested on mortar and masonry, glass, aluminum, and other surfaces |
The class is the number I look at first for anything outdoors. It's what decides whether a joint will survive its seasonal movement; our joint design guide shows how to do the math. Water-based latex sealants have their own standard, ASTM C834, and generally have lower movement capability than C920 sealants. A tube that lists no standard at all isn't necessarily bad, but you're taking its word for it.
How much is in there
The standard cartridge holds 10.1 fluid ounces (about 299 mL) and fits every standard caulk gun. You'll also see small squeeze tubes holding a few ounces for quick fixes, large 28-ounce cartridges (mostly adhesives and some sealants) that need a large-frame gun, and foil "sausage" packs for pros. More in the caulk gun guide.
How far does a standard tube go? A 10.1-ounce cartridge holds about 18 cubic inches. That's roughly enough, in a perfect world with no waste, to fill:
- about 97 feet of joint 1/8 inch wide by 1/8 inch deep,
- about 24 feet of joint 1/4 inch by 1/4 inch,
- about 12 feet of joint 1/2 inch wide by 1/4 inch deep, or
- about 11 feet of joint 3/8 inch by 3/8 inch.
Real jobs lose some to priming, tooling, and the stuff left in the tube, so buy a little extra. The tube calculator will do the math for your exact joints.
Color and clarity
White, bright white, almond, biscuit, gray, black, and a rainbow of grout-matching shades. A few things to know:
- Clear latex goes on white. Water-based "clear" caulk looks milky in the tube and turns clear as it dries. A thick bead can take days. Don't panic; wait.
- Clear silicone is clear from the start, more or less. "Translucent" means frosty rather than glass-clear.
- Sanded caulk has a grit texture to blend with sanded grout; grout makers often sell caulk color-matched to their grout lines. See our sanded caulk review.
- Paintable white is often the best choice for trim, since paint will cover it anyway.
E. The date code
Somewhere on the tube, often near the back or on the base, there's a lot number and usually a use-by or manufacturing date. It matters. Silicone past its date may cure slowly or not at all, and water-based caulk can thicken or separate. Many water-based tubes also say to protect them from freezing; a tube that froze in the garage may be ruined. If a store's shelf stock looks dusty, check the date before you buy.
The warnings
Read them. Some polyurethanes contain isocyanates, which call for good ventilation and gloves. Solvent-based products may be flammable and strongly scented. Products sold in California carry Proposition 65 warnings. Every manufacturer publishes a Safety Data Sheet with the details, usually on their website.
Karen's cheat sheet
| The job | Look for these words |
|---|---|
| Tub, shower, sink | "100% silicone," "kitchen and bath," "mold-resistant" (bathroom guide) |
| Painted trim and baseboards | "Acrylic latex," "paintable," "interior" (bedroom guide) |
| Siding and exterior trim | "Exterior," "elastomeric" or "polyurethane" or "hybrid," "ASTM C920," Class 25 or better (exterior guide) |
| Window and door perimeters | "Window and door," ASTM C920, a movement class, "paintable" if needed (windows guide) |
| Driveway and slab joints | "Polyurethane," "self-leveling" or Grade P, Use T (garage guide) |
| Gutters and flashing | "Butyl," "gutter and flashing," or a roof sealant rated for metal (roof guide) |
Or skip the squinting and let the Caulk Finder read the aisle for you.