At 1.01 fluid ounces, 30 milliliters is almost equivalent to a single ounce; the two are interchangeable except in applications that require exact measurement. There are 29.6 milliliters in a fluid ounce.
It can be difficult to remember the exact conversion factors between metric volume units and U.S. customary volume units, so the close resemblance between 30 milliliters and 1 fluid ounce can act as a useful starting point for approximate conversions. For example, it only takes some quick multiplication to determine that there are roughly 240 milliliters in an 8-ounce cup. Similarly, 1 liter is roughly equivalent to 1 quart.