Ability scoreModifier
1-5
2–3-4
4–5-3
6–7-2
8–9-1
10–11+0
12–13+1
14–15+2
16–17+3
18–19+4
20–21+5
22–23+6
24–25+7
etc…

Ability modifiers can be expressed as:
$$
mod = \frac{score}{2} - 5
$$