Is there an Ability Modifier equation?

Question

Is there an equation to determine the Ability Modifier?

I'm not talking about the chart that lists

• 1 = −5
• 2-3 = −4
• 4-5 = −3
• 6-7 = −2
• etc.

I mean a legit equation that, if I somehow misplaced this information, I could determine what the modifier is based on the score?

Answer 1

The formula is simply:

$$ \frac{\mathrm{ability} - 10}{2} $$

with the result rounded down, if it contains any halves.

Answer 2

I know this is old but I think the current answers are close but slightly inadequate. I think the formula itself (rather than comments/descriptions) needs to account for rounding somehow, so here is an actual formula (in Google Sheets or Microsoft Excel syntax) that does work, and I feel is human-readable:

=(ROUNDDOWN(A1/2) - 5)

Where A1 is the ability score (or cell).

You could also use truncation, e.g. TRUNC(A1/2,0)-5, casting, e.g. INT(A1/2)-5, or the floor function, e.g. FLOOR(A1/2)-5 to effectively turn the decimals into integers. I feel like ROUNDDOWN is a bit more self-explanatory.

Mathematically, the above formula would be expressed using the floor function notation:

$$\left\lfloor\frac{a}{2}\right\rfloor - 5$$

Where \$a\$ is the ability score.

Answer 3

It's quite simple if you do a different equation like this:

$$\left(\frac{\text{score}}{2}\right) - 5=\text{Modifier}$$

Answer 4

The Euclidean quotient of Ability divided by 2, remainder discarded, minus 5

Euclidean division is the process of dividing two integers to produce an integer quotient and an integer remainder. For example, 11 div 2 yields a quotient of 5 and a remainder of 1, where div signifies Euclidean division.

Answer 5

I think it's already been stated, but the method I use is round the base number down to the nearest even (17->16) divide by half (16/2 = 8) and then subtract 5 (8-5=3)