To dice an onion, cut it in half through the root, peel it, and place one half cut-side down. Keep the root end attached while making evenly spaced lengthwise cuts toward it, then slice across those cuts to release cubes. A stable cutting board, a sharp chef’s knife, and curled fingertips give you more control. Stop before the small root end becomes unsafe to hold.
What you need
- A firm onion without soft or moldy areas
- A sharp chef’s knife that feels comfortable in your hand
- A stable cutting board
- A damp towel or nonslip mat to place under a board that moves
Wash your hands, rinse the onion under running water before cutting, and dry it. The FDA advises rinsing produce even when you plan to remove the peel, because cutting can transfer contamination from the outside to the inside. Do not wash produce with soap, bleach, or detergent.
How to dice an onion step by step
- Trim the stem end. Cut off the pointed stem end, but leave the hairy root end intact. The root helps hold the layers together.
- Halve it safely. Stand the onion on the flat trimmed end, then cut straight down through the root. Place each half cut-side down so it cannot roll.
- Peel it. Remove the papery skin and any tough or damaged outer layer. Discard it.
- Make lengthwise cuts. Point the knife toward the root and make parallel cuts from the stem side toward—but not through—the root. Space them according to the dice size you want.
- Make optional horizontal cuts. For a large onion or very fine dice, make one or two careful cuts parallel to the board, again stopping before the root. Because onion layers already separate vertically, many cooks omit this step for a medium dice.
- Cut across. Turn the knife perpendicular to the lengthwise cuts and slice from the stem end toward the root. The onion falls into diced pieces.
- Finish safely. When the remaining root section becomes too small to hold with curled fingertips, set it flat and chop only what you can control. Discard the hard root.
Choose the dice size
| Result | Approximate spacing | Useful for |
|---|---|---|
| Large dice | About 3/4 inch | Roasting, skewers, chunky stews |
| Medium dice | About 1/2 inch | Soups, sautés, sauces |
| Small dice | About 1/4 inch | Salsas, fillings, quick-cooking mixtures |
The measurements are targets, not a reason to rush. Consistent spacing matters more than mathematical perfection because similarly sized pieces cook at a similar rate.
Knife control that makes the cut safer
Virginia Cooperative Extension recommends holding food with the fingertips curled inward in a “claw” shape and keeping the thumb behind them. Keep the board from sliding and cut at a pace you can control. A dull blade can require more pressure and is not a substitute for careful technique.
Some demonstrations place a flat palm on top of the onion for horizontal cuts. If you cannot keep the blade level and well below your hand, skip the horizontal cuts. The natural onion layers still allow a useful dice after lengthwise and crosswise cuts.
How to reduce tears
Chilling the onion for about 30 minutes can reduce eye irritation, according to Montana State University Extension. Ventilation also helps move irritating vapors away from your face. Do not trade control for a gimmick: slippery soaking, cutting while looking away, or rushing is less safe than pausing to rinse your eyes and reset.
Store the diced onion
Use it promptly or refrigerate it in a covered container. FoodSafety.gov advises refrigerating produce within two hours after it is cut, peeled, or cooked—or within one hour when it has been exposed to temperatures above 90°F. Clean the knife, board, and counter with hot soapy water, especially if the surface has also held raw meat, poultry, seafood, or eggs.
Related CurioBeacon answers
For another cooking term, see What Is a Binder in Cooking?. For a drink technique, read How to Make Cold Foam?.
Sources
Checked August 22, 2026.
