Winter kills cats through three routes — chill, dry air and swallowed poison — and snow is not one of them. Run the 8 checks below before the first night the indoor temperature drops under 20 °C, the point where a healthy adult cat starts losing heat faster than it can replace it.
Why 20 °C is the number that matters
A cat's thermoneutral zone runs roughly 30 °C to 38 °C. Inside that band the body spends almost nothing on heating itself. Below an ambient 20 °C, a lean adult cat has to lift heat production by 20 % to 30 % just to hold a core temperature of 38.1 °C to 39.2 °C, and it pays for that with calories it needed for something else.
Three groups lose the margin earlier: kittens under 6 months, cats over 12 years, and any cat under 3 kg. For them the working floor is 22 °C to 24 °C, not 20 °C. Measure where the cat sleeps, 10 cm to 15 cm above the floor, not at eye level. A thermometer on a shelf reads 3 °C to 5 °C warmer than the tile beside a balcony door on the same night, and the cat picks the tile. Take readings at 6 a.m. and 10 p.m. for 3 nights; the lowest number is the one you plan against.
Risk class 1: cold and moving air
Still air at 19 °C is tolerable. The same air moving at 0.2 m/s feels 4 °C to 6 °C colder, because the draught strips the thin layer of warmed air inside the coat. That is why the fix is rarely a bigger radiator: it is a 60 cm to 90 cm barrier at floor level, a door snake, and closing a gap under a balcony door that a 2 cm finger can slide into.
Bedding needs 4 cm of loft, not a folded towel 1 cm thick. A heated bed should hold 30 °C to 35 °C at the surface, be closed on 3 sides, and be switchable off at night so a cat that cannot stand up easily is not trapped on a hot pad. Offer 2 warm spots in different rooms, because the second one is what a bullied cat actually uses.
Risk class 2: air under 30 % humidity
Heating outdoor air from 2 °C to 21 °C drops relative humidity by roughly 5 percentage points per degree, so a 45 % outdoor reading becomes 20 % to 25 % indoors within a week of the heating going on. Under 30 % the nose dries, the coat throws static with every touch, and water intake creeps up while nobody watches. Aim for 40 % to 60 %; a $12 to $25 meter removes the guesswork.
A unit rated 2 L to 4 L per 24 hours covers 20 m² to 30 m². Refill with filtered water, rinse the tank every 3 days and descale every 14 days, because a dirty tank aerosolises bacteria straight into a cat's airways. Set out 3 water stations, 1 of them away from the food bowl, and expect 40 ml to 60 ml per kg per day — 180 ml to 270 ml for a 4.5 kg cat. The humidifier checklist covers placement and tank cleaning in more detail, and the static problem has its own fix in why winter shocks everything.
Risk class 3: the poisons winter adds
Ethylene glycol kills within 24 hours. The lethal dose is about 1.5 ml per kg, which for a 4 kg cat is 6 ml — under 2 teaspoons of spilled antifreeze, and it tastes sweet enough that cats lap it. Any spill on a garage floor gets soaked up and the patch rinsed with 5 L of water, never left to dry into a film.
Ice melt is second. Most products are chloride salts, and a cat that walks through them and then grooms takes in enough to vomit for 2 to 3 days. Wipe the paws with a damp cloth at 35 °C to 38 °C within 5 minutes of coming inside, dry between the pads, and switch to grit if the cat goes out daily. Rodent bait, whose use peaks from November to February, is third: 1 block is enough to cause internal bleeding in a 4 kg cat, and bait stations belong in a closed 20 L bin, not a cupboard a cat can nose open. Numbers for the common household toxins are listed in the toxicity lookup.
The 8-item checklist
| # | Check | Target number | Recheck every |
|---|---|---|---|
| 1 | Floor-level temperature in the coldest room | 20 °C, or 22 °C for old, young or small cats | 3 days, then weekly |
| 2 | Draught speed at the sleeping spot | under 0.2 m/s | 7 days |
| 3 | Bedding loft and heated bed surface | 4 cm loft, 30–35 °C surface | 1 night |
| 4 | Relative humidity | 40–60 %, never under 30 % | daily, 6 a.m. |
| 5 | Water stations and daily intake | 3 stations, 40–60 ml/kg | 24 hours |
| 6 | Antifreeze, ice melt, rodent bait storage | 0 open containers, 1.5 ml/kg lethal | season start |
| 7 | Weight on a kitchen scale | ±50 g of baseline | 3 days |
| 8 | Resting breathing rate while asleep | under 30 breaths per minute | weekly |
What to measure in the first 14 days
Weigh the cat every 3 days on a kitchen scale, same time of day, with a ±50 g tolerance. A 200 g drop across 2 winter weeks is a vet call, not a diet win. For water, fill a measured jug and subtract what is left after 24 hours; the target is 180 ml to 270 ml for a 4.5 kg cat. Count litter clumps per 24 hours — 2 to 4 is normal, and 1 or fewer for 2 days running is a flag for both drinking and urination.
Then count the resting respiratory rate: while the cat sleeps, count breaths for 30 seconds and multiply by 2. Under 30 per minute is normal, 30 to 40 needs a call within 48 hours, and over 40 at rest is same-day. Gums should be pink; pale grey or white is not.
When it stops being a checklist
Core temperature under 37.5 °C or above 39.5 °C, breathing over 40 per minute at rest, no food for 24 hours, or any suspected antifreeze contact means a same-day visit, not a thermostat adjustment. Hypothermia in a cat under 3 kg can move from shivering to unresponsive in under 2 hours, so start warming on the way to the clinic with a 38 °C to 40 °C blanket.
Do these 5 things tonight
- Read the thermometer 10 cm above the floor at 6 a.m. and bring the coldest room to 20 °C, or 22 °C if the cat is over 12, under 6 months, or under 3 kg.
- Block the 2 worst draughts with a 90 cm barrier and a door snake, and seal the balcony gap.
- Set the humidifier to 45 % and place it 1 m from any wall, then rinse the tank in 3 days.
- Rinse the garage floor and move antifreeze, ice melt and bait into one closed 20 L bin.
- Weigh the cat, write the number on the fridge, and repeat in 3 days; note the water jug total at the same time.
Frequently asked
Is 20 °C really too cold for a cat indoors?
For a healthy adult cat in good weight, 20 °C is workable but no longer free: it has to raise heat production by 20 % to 30 % to hold a normal core temperature of 38.1 °C to 39.2 °C. Kittens under 6 months, cats over 12 years and cats under 3 kg should sit at 22 °C to 24 °C, measured 10 cm above the floor where they actually sleep.
How often should I refill or clean a winter humidifier?
Refill daily with filtered water, rinse the tank every 3 days and descale every 14 days. A unit rated 2 L to 4 L per 24 hours covers 20 m² to 30 m². Keep the room at 40 % to 60 % relative humidity and never let it fall under 30 %, because a dirty tank aerosolises bacteria directly into the airways.
What is the actual lethal dose of antifreeze for a cat?
About 1.5 ml per kg of ethylene glycol, so 6 ml for a 4 kg cat, which is less than 2 teaspoons. Signs can appear within 30 minutes to 12 hours and death can follow inside 24 hours, so any suspected contact is an immediate clinic visit, not a wait-and-see. Rinse spills with 5 L of water and store containers sealed.