Sleeping Bag Temperature Calculator

A bag's headline number can leave you cold — literally. Tell us the overnight low you expect, the kind of sleeper you are and your setup, and we'll tell you the ISO 23537 Comfort rating to actually shop for, with a plain-English guide to what the ratings mean. Free, no sign-up.

1. Coldest overnight temperature you expect (°C)
2. What kind of sleeper are you?
3. Your setup (both affect how warm you'll be)

Sleeping bag temperature questions, answered

What is the ISO 23537 rating, and why does it matter?

ISO 23537 (which replaced EN 13537) is a standardised lab test that gives a bag three figures — Comfort, Limit and Extreme — so you can compare bags from different brands on the same basis. A bag "rated to 0°C" is meaningless unless you know which figure that is. Look for the Comfort rating and treat marketing headline numbers with caution.

Why shop to the Comfort rating and not the Limit?

The Limit rating assumes a warm sleeper, curled up, on the edge of comfortable — a recipe for a cold, broken night if conditions are worse than forecast. The Comfort rating gives you headroom for being tired, hungry, dehydrated or simply a colder sleeper than the lab's test subject. Buying to Comfort is buying a margin of safety.

Does my sleeping pad really change the bag I need?

Yes — a great deal. A sleeping bag insulates the air around you, but the insulation under you is crushed flat and does almost nothing; the pad does that job. On a thin pad you lose heat straight into the ground, so you need a warmer bag to compensate. A pad's R-value measures this: roughly R 3+ suits three-season use, R 5+ for cold nights.

How accurate is this calculator?

It's a guide, not a guarantee. It applies sensible margins on top of the ISO Comfort rating for sleeper type, shelter and pad — but wind, humidity, your clothing, food and hydration all matter on the night. Use it to narrow your shortlist, then read the bag's full ISO figures and real-world reviews before you buy.