Free tool
Sleeping Bag Temperature Calculator
From the forecast low to the comfort rating you should shop by.
Shop for comfort rating of
-7 °C
Bag class
Winter
The maths, step by step
- Forecast low-2 °C
- Sleep style offset0 °C
- Shelter offset-2 °C
- Planning margin−3 °C
Pair it with
R-value 4 or higher sleeping mat
Below about 5 °C most heat is lost into the ground, not out of the bag.
Shop the comfort column on ISO / EN labelled bags, not the limit column. The required rating is a floor — a bag rated colder always fits, and you can vent its warmth on milder nights.
All calculations run on our servers from the values you enter — nothing is stored.
Please note — Results are planning estimates for a sleep system, not a guarantee of warmth. Cold exposure is a genuine health risk for very young, older or unwell campers, so err toward the colder rating and re-check the forecast on the morning you leave. This tool does not replace manufacturer ratings or medical advice for anyone sensitive to cold.
Sleeping bags are sold with three different temperature numbers, and picking the wrong one turns a great trip into a long cold night. This tool cuts through the labels. You enter the coldest temperature you expect. You say how you sleep and where. It returns one number to shop by.
That number is a comfort rating in Celsius. A bag labelled at or below it should keep an average adult sleeping, not merely surviving. The extra margin matters because forecasts skew warm on calm nights, and because a chilled body at midnight does not negotiate.
The tool also flags the test standard behind bag labels and the mat underneath the maths. Most cold nights in Australian camps are lost through the ground, not through the zip. A warm bag on a cold mat is a warm sleep system on paper only.
How the calculation works
Start from the forecast low, not the forecast. A night described as mild at eight degrees can slide to three with clear skies and still air. Cold drains away in valleys inland, and the number you need is the bottom of the curve.
Your sleep style adjusts that. Warm sleepers, who kick a doona off at home, can live with a rating three degrees higher than the low. Cold sleepers need three degrees lower. Average sleepers take the number as it comes.
Shelter adjusts again. A tent is the neutral case: it blocks wind but does nothing about heat lost to the ground. A swag sits lower and runs colder, so it costs two degrees. Under open stars, radiating heat into a clear sky costs three.
Then the tool subtracts a flat three-degree margin. That cushion covers a forecast that misses, fill that has drifted down with age, and the honest gap between a test chamber and a camp. It is not padding. It is the difference between sleeping and enduring.
A worked example
Take a June night in the Southern Highlands. The forecast calls for a low of two degrees, you sleep cold, and you sleep in a tent. The maths: two, minus three for the sleep style, nothing for the shelter, minus three for the margin.
The required comfort rating is minus four degrees. A bag sold as a zero-degree crossover will not cover it. You want a winter bag labelled to minus four comfort, or a limit-rated minus-nine bag if you sleep a curled, zipped sleep.
Run the same night for a warm sleeper under open stars. The forecast did not change, but the required rating lands at minus one degree. Sleep style and shelter move the answer as much as the weather does, which is why both get their own input.
One more check before you buy. Read the low for the road you are taking, not just the first night. A route that climbs from coast to tablelands can drop ten degrees between Friday and Sunday, and a bag chosen for the start of a trip can be the wrong bag at the end of it.
How to use this calculator
Check the forecast for the coldest valley you will sleep in, not the nearest town. Towns sit warmer than surrounding camps, and river gullies drain cold air overnight. A camp ten minutes off the highway can run five degrees colder at dawn.
Choose your shelter honestly. Most Australian cold-country camping happens in a swag or a tent with the fly on, because of wind or dew. Sleeping under stars is a lovely idea with a big bag and a bad one with a summer quilt.
Then read the answer as a shopping floor, not a target. A bag rated warmer than the required number is the wrong buy at any price. A bag rated colder costs a few grams and always leaves you the option of opening a zip.
Interpreting your result
The result is a comfort rating. Under the EN 13537 standard and its ISO successor, comfort is the warmer of the two labelled numbers. It is tested for a reference adult woman. Limit is the lower figure, tested for a reference adult man sleeping curled.
If a bag shows only one number, ask which one it is. A limit label printed as three degrees can mean a comfort of eight. Cold sleepers and side sleepers should shop the comfort column every time, even when the marketing shouts the limit figure largest.
The mat note is not decoration. Below about five degrees, an R-value of four or higher is where most downward heat loss stops. A minus-five bag on a thin foam mat is an expensive way to stay cold, because the ground wins the exchange.
The one-number rule
When torn between two bags, take the colder rating. You can vent a warm bag. You cannot unzip your way into extra fill.
Accuracy, assumptions and limits
Every lab test measures a bag on a dummy in a chamber, over a simulated insulating base. Real ground, real drafts and real damp all cut the number. The three-degree margin in this tool exists exactly because the tested figure flatters the product.
Damp is the biggest cut. Canvas tents weep condensation in still weather, and a bag that absorbs some of it loses loft. Loft is where the warmth lives. Dry the bag out on sunny mornings and keep it off the tent floor at night.
Fill degrades too. A down bag held compressed in a swag or van rack for years will not regain its tested puff. Synthetic fill shrugs off abuse better and holds its rating more honestly, which matters more on long trips than gram-counting does.
This model covers sleeping, not sitting up reading at five degrees before dawn. Dress for the wake hours separately. The rating assumes a fed, dry body inside a properly used bag, and nothing finer than that.
The formula
Required comfort rating = forecast low + sleeper offset + shelter offset − 3 °C margin
- Low
- The coldest temperature forecast overnight, not the evening reading. Use the bottom of the curve.
- Sleeper
- Warm sleepers add 3 °C, cold sleepers subtract 3 °C, average sleepers take no offset.
- Shelter
- Tent is neutral, swag minus 2 °C, sleeping under open stars minus 3 °C.
- −3 °C
- A flat planning margin for forecasts that miss, ageing fill and damp nights.
Forecast low to bag class
| Expected low | Typical bag class | Notes |
|---|---|---|
| 15 °C and up | Summer weight, comfort 10+ | Tropical dry season and mild coastal summers |
| 8–15 °C | Three-season, comfort 5–10 | Most spring and autumn camping in southern states |
| 2–8 °C | Cold-weather, comfort 0–5 | Tableland winters; swag sleepers drop a class |
| -4–2 °C | Winter, comfort -5 to 0 | Frost-country nights, and check the mat rating too |
| Below -4 °C | Expedition class, comfort below -5 | High country and deep inland winter; real margin territory |
Key terms
- Comfort rating
- The lab-tested temperature at which a reference adult woman sleeps warm, per EN 13537 and ISO 23537-1.
- Limit rating
- The lower tested bound for a reference adult man sleeping curled. Three to five degrees colder than comfort, and the number marketing likes to quote.
- R-value
- A mat's resistance to heat loss into the ground. Higher insulates better; four or more for cold country.
- Loft
- How puffy the fill is when it traps still air. Warmth lives in loft, and damp and compression kill it.
- Radiative cooling
- Heat lost upward to a clear night sky. It is why frost forms on cars under stars, and why open-air sleeping runs colder than the forecast.
Pro tips
- Shop the comfort column, not the limit column, especially if you sleep cold or sleep on your side.
- Spend on the mat before the bag. Below five degrees, the ground wins most arguments.
- Eat before bed. A fed body sleeps warmer, and a cold one burns through its fuel in a short night.
- Dry the bag on sunny mornings. Damp fill is the quiet thief of a rated number.
× Common mistakes
- Reading a limit label of three degrees as a three-degree night and arriving with a summer bag.
- Choosing the exact forecast low with no margin, then fighting the condensation.
- Rating the bag and forgetting the shelter. Under stars, everything runs colder than the label.
Frequently asked questions
What is the difference between comfort and limit ratings?+
Both come from the same lab test, at two temperatures. Comfort is where a reference woman sleeps warm. Limit is where a reference man sleeps curled and does not wake from cold. In practice, comfort is the number most campers should shop by, and it runs three to five degrees warmer than the figure printed largest on the tag.
Are Australian sleeping bags sold to ISO standards?+
Most quality brands test to EN 13537 or its successor ISO 23537-1 and show both comfort and limit. Cheap bags sometimes quote an in-house rating with no standard behind it. If one number appears with no standard named, treat it as marketing and apply this tool's margin generously.
I sleep in a swag. Do I really need a colder rating?+
Yes, by about two degrees. A swag floor sits closer to the ground than a tent footprint, and canvas gives almost no insulation. Pair that with a mat of at least R-value four on cold ground, and the extra rating earns its keep on every frost morning.
Is a quilt better than a full bag?+
For warm sleepers in mild weather, yes, because venting is easier and weight is lower. For cold nights, a full bag with a hood traps heat a quilt cannot. This tool answers for a bag. If you run a quilt, carry an extra couple of degrees of margin on top of its result.
What about humidity in the north?+
Tropical nights rarely approach freezing, so the bag maths is easy up north. A ten-degree summer bag covers almost any dry-season camp. Humidity matters for storage instead: fill that stays damp in a wet-season van loses loft over time, which shows up later as a colder real rating than the label claims.
Does a bag liner add warmth?+
A liner adds perhaps one or two degrees at best, and cotton ones absorb moisture quickly. Carry one for hygiene rather than heat. When the required rating drops below zero, the real answer is more fill, not more socks, and a proper mat underneath matters before either.
How we built this calculator
- Written by
- Track & Canopy Editorial Team — Gear & Sleep Desk
- How we calculate
- Required ratings follow the EN 13537 and ISO 23537-1 comfort test definitions, adjusted with flat sleeper, shelter and margin terms that reflect published manufacturer guidance on choosing a rating against expected lows.
- Sources & standards
- Last reviewed
- 15 October 2026
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Track & Canopy — Practical gear advice for Australian conditions. Estimates are planning aids, not engineering advice.