To convert fluid ounces to cups (fl oz to cup), multiply by 0.125. 1 fluid ounce equals 0.125 cup, about 5.9 times as much as a teaspoon.
| fluid ounces | cups |
|---|---|
| 1 fluid ounce | 0.125 cup |
| 2 fluid ounces | 0.25 cup |
| 5 fluid ounces | 0.625 cup |
| 10 fluid ounces | 1.25 cup |
| 25 fluid ounces | 3.125 cup |
| 50 fluid ounces | 6.25 cup |
| 100 fluid ounces | 12.5 cup |
| 250 fluid ounces | 31.25 cup |
| 500 fluid ounces | 62.5 cup |
| 1000 fluid ounces | 125 cup |
The factor is 0.125000, which is derived rather than defined, so any decimal you see has been rounded somewhere.
For mental arithmetic the factor is close enough to 0.125 to use directly: multiply fluid ounces by 0.125 and the answer is right to within a rounding error.
Going the other way, one cup is 8 fl oz, which is usually the easier direction to work in. Because the forward factor is small, watch the decimal place rather than the digits: a factor-of-ten slip is the common error, not a rounding one.
To anchor the scale: 10 fluid ounces is about 1.25 cup.