Quick answer

Choose TOG by the temperature of the room where your baby sleeps, not by the season on the calendar. As a starting point: 0.5 TOG for rooms around 75-80°F, 1.0 TOG for roughly 69-74°F, 1.5 TOG for about 66-68°F, and a 2.5 TOG sleep sack for roughly 61-65°F. If your nursery swings more than a few degrees overnight, buy for the coolest temperature your baby will actually sleep in and adjust the base layer underneath.

Start with a thermometer, not the forecast

The single most useful purchase before a sleep sack is a cheap room thermometer. TOG ratings describe how much warmth a garment traps, so the rating only works when you know the temperature it is trapping warmth against. A nursery that runs 70°F in January needs a lighter sack than a drafty 63°F room in the same month.

Check the temperature at cot mattress level in the middle of the night, not just at bedtime. Many rooms are warmest at 7 p.m. and coolest around 3-5 a.m., which is exactly when a too-light sack becomes a problem.

  • Read the thermometer at mattress height, away from radiators, vents, and direct sun.
  • Note the overnight low, not the bedtime reading, before choosing a TOG.
  • Re-check after any change to heating, air conditioning, or window coverings.

A practical TOG-to-temperature starting chart

Use these ranges as a starting point and adjust for your baby's own warmth. Two babies in the same room can need different TOG ratings, so treat the chart as a first guess and check your baby's chest and back rather than hands or feet, which normally feel cooler.

The base layer matters as much as the TOG. A long-sleeve cotton bodysuit under a 2.5 TOG sack adds meaningful warmth, while a short-sleeve bodysuit or just a diaper under the same sack runs cooler. Change the layer before you change the sack.

  • 0.5 TOG: roughly 75-80°F, typically a summer or well-cooled room.
  • 1.0 TOG: roughly 69-74°F, the most common year-round nursery range.
  • 1.5 TOG: roughly 66-68°F, useful for spring, autumn, and air-conditioned summers.
  • 2.5 TOG: roughly 61-65°F, the typical choice for cold winter rooms.

What TOG for a winter baby, and when to switch down

For winter, a 2.5 TOG sleep sack is the usual answer when the nursery sits around 61-65°F. If your home stays above 68°F in winter, a 2.5 TOG sack plus a warm bodysuit can be too much, and a 1.0 or 1.5 TOG is the better call.

Switch TOG when the overnight low moves out of the band you bought for, not on a fixed date. A simple rule: if your baby wakes sweaty or hot on the chest and back, go lighter; if they wake cool and unsettled in a room within the chart range, go warmer or add a layer. Never add loose blankets to compensate, since a sleep sack's job is to replace them.

  • Move from 2.5 TOG to 1.5 TOG when the overnight low stays above about 65°F.
  • Move from 1.5 TOG to 1.0 TOG when the overnight low stays above about 68°F.
  • Move from 1.0 TOG to 0.5 TOG when the overnight low stays above about 74°F.
  • Change one variable at a time and give it two or three nights before judging.

Fit, fabric, and safety checks that outrank TOG

A sleep sack only works if it fits. The neck opening should be snug enough that the sack cannot ride up over the chin, and the armholes should let arms move freely without letting the sack slip off a shoulder. If the sack bunches around the face or your baby can pull it up, size down or change style.

Length matters too. A sack that is too long can trip a crawling or pulling-up baby during awake time, and one that is too short restricts leg movement. Check the manufacturer's height and weight range rather than guessing from age, and replace the sack when the shoulders no longer sit correctly.

Fabric choice is a comfort decision, not a safety rating. Look for breathable materials and avoid adding a hat, thick quilt, or extra blanket inside the sack. A firm, flat mattress with a fitted sheet and a correctly fitted sleep sack remains the safe setup.

  • Snug neck, free arms, no bunching at the face.
  • Follow the brand's height and weight range, not just age.
  • No hats, quilts, or loose blankets layered inside or over the sack.

How many sleep sacks you actually need

Most families do well with two sacks in the current season's TOG: one in use and one in the wash. If your nursery crosses two temperature bands across the year, plan for one sack in each band rather than trying to stretch a single rating through every month.

If you are buying ahead, buy the next TOG band down rather than up. Babies tend to run warmer as they get bigger and more mobile, and a lighter sack can always be paired with a warmer base layer.

  • Two sacks per TOG band covers laundry day without panic.
  • Buy the lighter band ahead, not the heavier one.
  • Retire any sack that no longer fits at the neck or shoulders.

Decision rules for choosing and switching TOG

  • Room at 61-65°F: Choose a 2.5 TOG sleep sack and a long-sleeve bodysuit base layer. Check the chest and back at the overnight low. This is the standard winter setup for a cold nursery.
  • Room at 66-68°F: Choose 1.5 TOG, or 2.5 TOG with a light short-sleeve layer if your baby sleeps cool. The transition band where most switching mistakes happen.
  • Room at 69-74°F: Choose 1.0 TOG with a light bodysuit. This covers most heated and air-conditioned homes year-round. If you only buy one sack, start here.
  • Room above 74°F: Choose 0.5 TOG and a short-sleeve bodysuit or diaper only. Avoid heavy layers and hats. Summer and warm upstairs rooms.
  • Baby wakes sweaty or hot: Go one TOG band lighter or remove a base layer before changing anything else. Overheating is the signal to act, not to add a blanket.
  • Baby wakes cool and unsettled: Confirm the room is inside the band for your current TOG, then add a layer or move up one band. Change one variable at a time and re-check for two or three nights.

Products that fit this decision

  • WildBird CloudBlend™ Sleep Sack TOG 2.5 - Sparrow: WildBird CloudBlend Sleep Sack TOG 2.5 - Sparrow is a wearable blanket made with CloudBlend fabric, available in multiple sizes. This is the 2.5 TOG option for cold nurseries around 61-65°F, and the size options let you match the sack to your baby's height and weight rather than guessing.
  • Hubble Connected GoBaby AI Portable Pro: Hubble Connected GoBaby AI Portable Pro is a baby monitor that uses AI to support the monitoring experience. A monitor helps you check on a sleeping baby without opening the door and changing the room temperature, which keeps your TOG choice consistent through the night.

Who this approach fits

Best for

  • Parents whose nursery temperature changes across seasons or overnight
  • Families choosing between 1.0, 1.5, and 2.5 TOG sleep sacks
  • Caregivers who want a simple rule for when to switch TOG

Not for

  • Anyone looking for a single sack to cover every temperature without adjusting layers
  • Parents who cannot measure the overnight room temperature and prefer to follow a fixed seasonal schedule
  • Babies outside the height and weight range of the sack they are considering