MarkT:
The capacity will depend on the discharge rate - high discharge rates can reduce the apparent capacity markedly - though what counts as a high discharge rate depends on the chemistry. Because of this battery capacities are usually quoted at a specific rate (often the "10 hour rate").
Since OP is describing 1.5V batteries I thought I'd point out that Duracell has datasheets that show the Ah ratings of their batteries given different discharge rates. What MarkT said is absolutely correct, but, y'know, it's helpful to see a picture too 