Child eating hot lunch packed in a thermos

How to Keep Food Warm – Lunchbox Tips

If you've got a child who wants warm lunches at school and there's no microwave available you've probably looked into using insulated containers to pack their lunch. You might wonder though – do insulated lunch bags keep food warm enough? Do you need to invest in a thermos and feed them nothing but soup? We've gathered a few tips and tricks to help you keep your kids happy with warm meals.

Child eating hot lunch packed in a thermos
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Keeping Lunchbox Foods Warm

Plastic or metal lunch boxes don’t provide the proper amount of insulation to keep food hot or cold, and brown bags are even worse. However, using an insulated bag along with other sources of insulation or heat can do the trick.

Using a Thermos for Hot Lunch Foods

The USDA recommends using an insulated container like a thermos to keep hot foods above 140 °F in packed lunches. You can test out your own thermos at home to see how well it holds heat. Heat water to at least 165 °F, then leave the thermos at room temperature and check the water temperature after 2 hours to see if it's stayed above 140 °F.

To pack hot food into a thermos:

  • Boil water and pour into thermos.
  • Heat food until it reaches an internal temperature of 165°F.
  • Empty water from thermos, pack food, then tightly seal.
  • Keep thermos sealed until ready to consume to maintain a hot temperature

Tip: Foods such as soups, pasta with sauce, or stews do best staying warm in a thermos because they have more liquid and less air gaps. Using a neoprene sleeve such as those made for wide-mouth mason jars can add an extra layer of insulation.

Keeping Food Warm in an Insulated Lunch Bags

Over the years, insulated lunch bags have become increasingly popular, and some even include gel packs that can be heated at the base. Lava Lunch leads the way for holding hot lunches. Their lunch bags also include dividers for hot and cold food.

If you choose to go with an insulated lunch bag, you still need to thoroughly heat food before packing it. You can also wrap it in aluminum foil after it’s been cooked to seal in the warmth. Add an additional layer of insulation by wrapping it in a tea towel, napkins, or paper towel.

Unless you or your kid will eat the food right away, you need an insulated lunch bag to retain the heat long enough, so it’s still warm three to five hours later. Layers are incorporated in insulated lunch bags, and they’re the key to everything. Typically, each bag has an outer layer, an inner layer, and then a layer of insulation between the two.

Tip: Look for easy to clean fabrics. An inner surface that can be wiped clean with warm soapy water is ideal.

Heat Packs for Added Lunch Warmth

While the primary purpose of an insulated lunch bag is to keep your prepared food hot or cold, using a heat pack helps maintain the warmth. Some lunch bags come with them (such as the previously motioned Lava Lunch bags), but you can purchase heat packs on their own. They are affordable and easy to use. Heated gel packs have a flexible gel inside of their plastic pouch and can be heated in the microwave.

Tip: Most of the heated gel packs you can find are made to keep casserole's warm and are large because of this. Look for a size that's 7×10″ or smaller to fit in your lunch bag. This size should sit nicely under most bento boxes to keep the contents warm.

A Keep Food Warm Lunchbox?

One option that's useful to know about if you've got a child that really must have a hot lunch is self-heating lunch boxes like Hot Bento. These and other electric lunch boxes act as portable food warmers. They do tend to be more expensive, thus some might not want to risk sending them to school.

Extra Tips for Keeping Food Warm

  • Reheat food to 165 °F before packing into your insulated container. Reheating to this temperature helps prevent foodborne illness and the extra heat keeps the food warm during the hours between preparation and consumption.
  • Do not open the thermos once it’s been sealed because that will allow the heat to escape.
  • Wrap a tea towel around the thermos to add additional insulation. This also helps with separating hot containers and cold food such as juice boxes and fruit cups.
  • Add a heated gel pack or bottle to the insulated lunch bag to add extra warmth.
  • Always ensure hot and cold food are kept separated to the best of your ability.
  • Make sure that perishable food is cooked to a safe internal temperature and chilled quickly after cooking.
  • Test out your insulated lunch bag by preparing your own lunch and seeing how long it remains warm.


Using insulated lunch bags to keep food warm is helpful, but you should always take additional steps to ensure that the food stays as hot as it can and for as long as it can. You can use all the information provided here to help you keep your and your kid’s lunches warm and edible. Happy eating!

Alissa Zorn stands near a pond with an orange shirt on wearing a black button down over that.
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Alissa Zorn is an author, and founder of the website Overthought This. She's a coach and cartoonist passionate about helping people overcome perfectionism and shame to build authentic, joyful lives. Alissa is certified through the International Coach Federation and got her Trauma-Informed Coaching certification from Moving the Human Spirit. She wrote Bounceback Parenting: A Field Guide for Creating Connection, Not Perfection, and is always following curiosity to find her next creative endeavor.