About WhichTOG
A free calculator that turns a nursery's room temperature into a sleeping-bag TOG rating and clothing layers, for UK parents and carers dressing a baby for sleep.
WhichTOG is a simple, free tool: set the room temperature and it recommends a sleeping-bag TOG rating and the clothing to wear underneath it, based on published UK guidance. It has no accounts, no sign-up and no adverts — you set a temperature and get a recommendation.
How the recommendation works
The room temperature is placed into one of eight bands (below 13°C, 13–16°C, 16–18°C, 18–20°C, 20–22°C, 22–24°C, 24–27°C, and above 27°C), each with a published TOG chart range drawn from brand and NHS/Lullaby Trust guidance. Each band also has a baseline clothing level, from a nappy alone in the warmest band up to a long-sleeved bodysuit, sleepsuit and an extra warm layer in the coldest.
If you tell WhichTOG which sleeping bags and clothing items you actually own, it picks the bag you own that's closest to the band's ideal TOG, then adjusts the clothing to compensate for the gap: within 0.75 TOG of the ideal, the baseline clothing is left as it is; beyond that, a layer is added or removed for roughly every full TOG of difference. This is how the same bag can be used across a wider range of room temperatures, by changing what's worn under it.
Because overheating is the greater safety risk, the warm-side guard is stricter than the cold-side one. If the bag you own would be too warm for the room, the app switches to a lighter bag you own with extra clothing to compensate, shows the lighter TOG to aim for, or — if even your lightest bag would risk overheating — recommends no sleeping bag at all with a light layer instead. If your warmest bag would be too light for the room, it keeps that bag and adds clothing layers, with a warning that a warmer bag would be better.
Below 13°C and above 27°C the calculator shows an edge-of-range warning instead of a normal recommendation: above 27°C it suggests removing the sleeping bag entirely and cooling the room where possible; below 13°C it suggests warming the room toward the 16–20°C range recommended by the NHS and The Lullaby Trust.
A humid-room toggle adds an advisory note (humidity makes a room feel warmer and raises overheating risk) without changing the recommended TOG or clothing — it's a prompt to watch for signs of overheating, not a different calculation.
The "if it drops" and "if it rises" sections show what to switch to if the room cools or warms overnight, and — where location access is allowed — a local weather forecast estimates the room's likely overnight low from the outdoor forecast, since bedrooms typically sit a few degrees warmer than outside.
Sources
The temperature bands, TOG ranges and safe-sleep guidance are drawn from:
- NHS — Safe sleep advice for babies
- The Lullaby Trust — Safe sleep & room temperature
- John Lewis — Baby Sleeping Bags Guide
- M&S — TOG & room temperature guide
- Slumbersac — Understanding TOG ratings
- JoJo Maman Bébé — Sleeping bag & TOG guide
- aden + anais — Sleeping bag TOG chart
Each band in the calculator's "Sources" panel names which of these it draws its TOG figure from, and the NHS and Lullaby Trust guidance underpins the 16–20°C ideal room temperature used throughout.
How it's checked
An automated safety audit runs the recommendation engine against every combination of room temperature (10–30°C), sleeping-bag wardrobe and clothing wardrobe the app supports — 324,105 scenarios in total. It checks, for every one of them, that:
- the recommended bag is always one you selected (or, when it isn't, the app clearly flags it as an "aim for" recommendation rather than showing it as if you owned it);
- the clothing level recommended is always within the valid range;
- any bag more than 1 TOG off the ideal for the room always triggers an overheating or too-cold warning, so a dangerous mismatch is never left silent.
The audit is regenerated from the app's own code each time it runs, so it can't drift out of step with what the calculator actually does.
Privacy
WhichTOG has no accounts and doesn't collect personal data. If you use the local weather forecast, your location is sent only to the weather service to fetch the forecast — it is not stored.
Not medical advice
WhichTOG is a guide based on published room-temperature and TOG guidance, not medical advice. If you're unsure how to dress your baby for sleep, or have any concerns about their health or sleep safety, speak to your health visitor or GP.
Last reviewed: 22 September 2026 — sources re-checked against current NHS and Lullaby Trust guidance.