What Is Formula-Based Validation?

That's where Custom validation comes in. Instead of picking a type from the Allow dropdown, you choose Custom and write a formula directly. Excel evaluates that formula every time a value is entered — if it returns TRUE, the entry is accepted; if it returns FALSE, it's rejected.

How It Works

1. Select the target cells;
2. Open Data Validation → Settings;
3. In Allow, choose Custom;
4. In Formula, enter any logical expression that returns TRUE or FALSE;
5. Click OK.

The formula is evaluated relative to the first cell in your selection — exactly like a conditional formatting rule. If you select H2:H51 and write =H2>0, Excel automatically adjusts it to H3>0, H4>0 and so on down the column.

Customer Email Example

The Customer Email column should always contain an @ symbol — that's the minimum check for a valid email format. No built-in rule covers this, but a custom formula does it in one line:

=ISNUMBER(FIND("@",G2))

How it works:

• FIND("@",G2) searches for @ in the email cell — returns its position if found, an error if not;
• ISNUMBER() converts that to TRUE (found) or FALSE (not found);
• Excel accepts the entry only if the result is TRUE.

What Makes a Valid Custom Formula

• Must return TRUE or FALSE (or 1/0);
• Can reference other cells in the same row;
• Can use any Excel function — IF, AND, OR, ISNUMBER, LEN, FIND, COUNTIF, and more;
• Cannot reference cells on other sheets directly in some Excel versions — use a named range as a workaround.

Task

Discount Only If Quantity Is Met

1. Apply a custom validation to the Discount % column:
   • Formula: =OR(H2>=3,J2=0);
   • Error style: Warning;
   • Error message: "Discount is only allowed for quantities of 3 or more";
2. Test by entering a discount of 10 with a quantity of 1 — observe the warning;
3. Change the quantity to 3 and re-enter the discount — confirm it is accepted.