Lunch Box Size Guide: How Much Capacity Do You Need?

What size lunch box do you need?

Start with what you eat, not with the number on the box. A lunch box that is too small means packing food against the lid or going hungry by mid-afternoon. One that is too big lets food slide around, takes up bag space and tempts you to overfill it.

For one adult meal, around 800 ml (27 oz) is a practical size. Go bigger if you eat one large meal in place of two, if you keep salad leaves loose so they stay crisp, or if you want space to separate components inside one container. Go smaller for snacks and sides.

• Snacks and sides: a small container of a few hundred millilitres, for fruit, crudités and dips, or a side salad.
• One adult meal: around 800 ml (27 oz), for a grain bowl, a pasta portion or a hearty salad.
• Large appetites and bulky food: around 1.2 litres (40 oz), for training days, leafy salads or a meal plus a snack in one box.

How much food does 800 ml (27 oz) actually hold?

800 ml is a little under a pint and a half, and in food terms it's one genuine adult meal. In practice, that looks like:

• A grain bowl: a base of cooked rice or quinoa, a palm-sized portion of protein and roasted vegetables, closed without squashing
• A dinner-sized portion of pasta with a thick sauce
• A salad with substance: leaves, a scoop of grains, chicken or chickpeas — with the dressing packed separately and added at your desk
• A deep serving of chilli or stew, with enough headroom that the lid closes cleanly

When should you choose a bigger or smaller lunch box?

Bulk and weight are different things. Leafy salads take up a lot of space for very little weight, so they suit a larger box, while dense food such as rice, pasta and stew packs a lot into 800 ml. If you eat big, an 800 ml (27 oz) container still holds a dense, filling bowl. What it isn't is a two-meal container: it's sized for one adult and one sitting.

Training days are the usual reason to size up. If you eat two prepared meals a day, two containers of the same size usually work better than one oversized box: portions stay consistent, and you can reheat one while the other stays in the fridge.

A mixed set of sizes tends to get used less than a matched set, because matched containers stack neatly and share one lid size.

How full should you fill a lunch box?

Leave space below the lid. A box packed to the rim squashes the food, leaves sauce on the rim where the gasket sits, and gives steam nowhere to go when you reheat.

In the microwave, take the lid off, or vent it according to the container's instructions: steam builds pressure in a closed container and can push the lid off or splash food. For the SEIDO lunch box, lid off is recommended; if the lid stays on, its steam vent must be fully open and clear, with food at least 1 cm (1/2 in) below the rim so nothing touches the lid.

For the freezer, leave more headroom still. Water expands by about 9 per cent as it freezes, so soups, stews and sauces filled to the brim push against the lid. Leave roughly 1 to 2.5 cm (half an inch to an inch) below the rim, and label each container with what's in it and the date.

Does the depth of a portion matter?

Yes, for cooling and for reheating. Shallow portions cool faster than one deep pot, which helps you get cooked food into the fridge within the one to two hours the Food Standards Agency advises; dividing a batch into smaller containers before chilling is part of the same advice.

In the microwave, an even layer heats through more evenly than a tall mound, because microwave heating is uneven and dense food heats from the outside in. Spread the food out, stir it partway, and check it is steaming hot all the way through before you eat.

How many lunch boxes do you need for a working week?

For one lunch a day, three containers is a practical number: one in your bag, one in the fridge and one in the wash. The Food Standards Agency advises eating leftovers within 48 hours or freezing them, so a weekend cook covers the first two days from the fridge; freeze the next portion and move it to the fridge to defrost the night before. A quick midweek cook covers the rest of the week.

For two prepared meals a day, plan on two containers for each day you cook ahead. More than that mostly adds cupboard clutter; fewer forces you to cook on a schedule that rarely survives a real week.

One large divided box or several separate containers?

Several separate containers often work better than one large divided box. Separate boxes keep a dressed salad apart from a hot dish, let you reheat only what needs heat and stack neatly in the fridge. A divided box suits food eaten cold, where nothing needs to go in the microwave on its own.

Why is the SEIDO lunch box 800 ml (27 oz)?

The size is deliberate: one container holds one complete adult lunch — a full grain bowl, a proper pasta portion or a salad with protein — so a batch divides cleanly into equal meals. The SEIDO Prep Series lunch box 3-pack is three identical 800 ml (27 oz) containers, each 304 (18/8) food-grade stainless steel with a glass lid in a polypropylene frame and a silicone gasket.

The container is lab-tested for microwave use with the lid off, in microwaves up to 1000 W, and never empty. It is lab-tested for freezer use and is dishwasher safe, hand wash recommended. It is also lab-tested for oven use as a container only; the lid never goes in an oven. The set is backed by a 5-Year Limited Warranty (lid parts: 1 year). If it fits your week, buy it on Amazon.