Standard Mouth with Flex Cap
Popular insulated steel bottle, but the standard Flex Cap is polypropylene - water sits against plastic threads, so it is not plastic-free despite the "steel bottle" reputation.
Plastic-free verdict: Minimal plastic contact
The body is 18/8 stainless steel, but the standard Flex Cap is recyclable polypropylene (#5) with a flexible TPU (thermoplastic urethane) strap and steel pivots. The cap threads and drink-contact surface are plastic, so when the bottle is full or tipped, water sits against and pours over polypropylene. Included here mainly as a corrective: many people assume a "stainless steel Hydro Flask" is plastic-free, but the lid puts plastic squarely in the water path. Hydro Flask does sell a separate all-steel Flex Cap that fixes this.
Verification: Manufacturer confirmed · reviewed 2026-07-05
What it's made of
| Part | Material | Food contact |
|---|---|---|
| bottle body (double-wall insulated) 18/8 (304) food-grade stainless steel | Stainless Steel (304 / 18-8) | Yes |
| Flex Cap body / threads recyclable polypropylene | Polypropylene (PP, | Yes |
| cap strap flexible TPU (thermoplastic urethane) carry strap, exterior | Plastic (other / unspecified) | No |
| strap pivots | Stainless Steel (304 / 18-8) | No |
Hydro Flask's signature insulated bottle. The steel body earns the brand its reputation, but the default Flex Cap is plastic where it counts - the interior and threads the water touches. Straw lids and Flex Sip lids are likewise largely plastic. It is a fine insulated bottle, but for a plastic-free water path you would need to buy Hydro Flask's separate stainless steel Flex Cap (steel-lined underneath), which converts it to roughly a silicone-only setup.
Pros
- Excellent insulation and durability; 18/8 steel body
- A separate all-steel Flex Cap accessory exists as a plastic-free upgrade
Cons
- Default Flex Cap is polypropylene - water contacts plastic at the cap
- Straw and Flex Sip lids are also mostly plastic
- Marketed as a "steel bottle" in a way that obscures the plastic lid
Notes
- Standard Mouth Flex Cap: polypropylene #5 body, TPU strap, steel pivots (brand-stated)
- Buying the separate Hydro Flask stainless steel Flex Cap moves the water contact to steel plus a silicone gasket
Categories: Water Bottles
Sources
Every material claim above is backed by these. This is the scattered info we centralized.
- manufacturer https://www.hydroflask.com/standard-mouth-flex-cap standard Flex Cap is recyclable polypropylene
- review https://www.bottlepro.net/hydration-blog/which-water-bottles-completely-avoid-plastic notes Hydro Flask's separate stainless steel Flex Cap is the plastic-free-in-the-water-path option; standard cap is not