Four hours, forty-eight hours, forty degrees

Quick answer: A closed refrigerator keeps food safe for about four hours. A full freezer holds for about 48 hours, or 24 if it's half full. After that, perishable food that's been above 40°F for two hours or more goes in the trash, however it smells.

Four hours. That's the clock, and most households burn through it by opening the door to check on things. Every peek lets cold air out and warm air in. The fridge isn't a refrigerator anymore at that point. It's a cabinet.

Everything below follows USDA and FoodSafety.gov guidance. It's not a place for improvising, and nobody gets a gold star for eating the questionable chili.

The three numbers

Four hours. That's the USDA figure for an unopened refrigerator. Keep the door shut and the clock runs slowly.

Forty-eight hours. A full freezer with the door closed holds its temperature about that long. Half full, plan on 24. A full freezer wins because frozen food keeps its neighbors cold. Empty space is just air waiting to warm up.

Forty degrees. Perishable food above 40°F for two hours or more is out. That's the line between "fine" and "discard," and it's measured with a thermometer, not a nose.

Buy the ten-dollar thermometer before you need it

An appliance thermometer is a small dial or digital gauge that sits in the fridge and another in the freezer. Get both. Check them when the power comes back, and they'll tell you what the food actually went through. The fridge should read 40°F or below and the freezer 0°F or below.

Without one, you're guessing. With one, you know whether the fridge crept to 38°F or sat at 55°F for three hours. A cheap one is one of the best-value preparedness purchases there is.

A food thermometer does the second job. When the power returns, check the temperature of the food itself. Anything still at 40°F or below is safe. The freezer follows the same rule. If food still has ice crystals or reads 40°F or below, you can refreeze it or cook it. Quality may suffer, but safety hasn't.

What goes in the trash

USDA says to discard these if they've been above 40°F for two hours or more:

• Raw and cooked meat, poultry and fish
• Hot dogs and lunch meat
• Milk, cream, yogurt and soft cheeses
• Eggs and egg dishes
• Casseroles, soups and stews
• Cooked pasta and pasta salad
• Cut melon, cut greens and cooked vegetables
• Cookie and biscuit dough
• Opened baby formula

Mayonnaise, tartar sauce and creamy dressings get a little more leeway. USDA says to throw them out if they were above 50°F for more than eight hours. Honestly, if the jar was warm for a while, a couple of dollars is cheap insurance.

Anything for an infant gets the strictest treatment. Check FoodSafety.gov for formula and breast milk rules. If someone in the house relies on refrigerated medication like insulin, call the pharmacist or doctor now, not mid-outage. Manufacturers publish how long each product holds up at room temperature, and the answer varies.

What stays

This list is better news:

• Hard cheeses like cheddar, Swiss and Parmesan
• Butter and margarine
• Peanut butter, jelly, mustard and ketchup
• Pickles and olives
• Opened vinegar-based dressings
• Whole fresh fruits and vegetables
• Bread, rolls and plain cakes (the cream-filled kind goes)

Those survive because they're low in moisture, high in acid or salt, or just don't spoil quickly. Know the list and you'll lose less food than the person who dumps the whole fridge in a panic.

Don't taste it

People love the sniff-and-nibble method. The bacteria that make you sick, including the ones behind most food poisoning, don't reliably change how food smells, looks or tastes. A steak can seem fine and still be a bad bet.

The USDA's rule is four words: when in doubt, throw it out. It's the one place where being stubborn costs you a hospital visit.

Be the person who says it kindly but doesn't budge. If an uncle swears the potato salad is fine, you can thank him for the opinion and put it in the bag anyway. You're feeding your family. Don't let anybody talk you out of the thermometer.

Making the hours last

Some of this happens before the storm.

• Set the fridge and freezer to the coldest settings when you know bad weather is coming.
• Freeze water in jugs or containers and tuck them into empty freezer space. They keep things cold now and give you drinking water later.
• Group frozen food together. Packed freezers stay colder.
• Keep a cooler and a couple of bags of ice handy. At the four-hour mark, move milk, meat and leftovers into the cooler with ice or frozen gel packs. Keep the food at 40°F or below.
• Dry ice works, with respect. USDA says 50 pounds should hold a full 18-cubic-foot freezer about two days. Wear gloves, never handle it bare-handed, and ventilate the room. It's cold enough to burn skin and it releases gas that can build up in closed spaces.

If you're running a generator to keep the fridge going, the placement rules in Twenty Feet From the House and Other Generator Rules matter more than the food does. Don't cook on a charcoal grill or camp stove indoors or in the garage either. Carbon monoxide kills people who've done everything else right.

When floodwater is involved

Floodwater changes the rules entirely. Discard anything that touched it, including food in containers that aren't waterproof, and anything in bottles or jars with screw caps or crimped lids.

Commercially canned food can sometimes be saved if the can is intact. Skip any that are bulging, leaking, rusted or badly dented. Wash the rest, then sanitize them. The CDC's method is to soak the cans for 15 minutes in a solution of one tablespoon of unscented household bleach per gallon of water. Then relabel them with a marker, because the paper labels won't survive.

For longer outages, What a Week Without Power Actually Takes covers the stretch where the fridge is already a lost cause and the pantry becomes the plan.

Take care of the people with the least cold

Somebody on your street is on a fixed income with a full chest freezer and no generator. Maybe it's an older relative, as covered in What Your Parents Need Before The Next Three-Day Outage. Knock on the door at hour three, not hour thirty.

Offer cooler space. Offer a bag of ice. Once the lights are back, offer to help sort the freezer, because nobody wants to throw away a winter's worth of food alone.

That's the old rule too: leave the neighborhood a little better than the storm left it.