B10 life represents the time period when 10% of the bearing population will fail, and 90% will survive. Life is not the same as MTBF. MTBF is usually calculated for some window of time (e.g., during system “useful life”) by dividing population unit hours by the number of failures occurring in that population. MTBF measured in this way may be constant over time, such as typical with electronics, which have few things that wear out, or may be increasing or decreasing over time. Weibull analysis can be used to determine the underlying failure characteristics (failure probability density function), and calculate the expected MTBF at any point in time (instantaneous MTBF). If you know the Weibull shape parameter, which determines the slope of the Weibull plot, and any other point such as B10 or the characteristic life (the point where 63.2% will fail), then you can calculate the MTBF at any other point using Weibull probability distribution relationships/equations.

Is MTBF bigger or smaller than B10?
This is like comparing apples to oranges – they are two different things. For example, think of tires on a car, which might have a life of 70,000 miles (i.e, 95% will fail by this point). The B10 life for a tire might be 40,000 miles. If you had many cars and computed “mean miles between tire failure” only for the window between 35,000 miles and 45,000 miles, you would compute a very high MTBF, probably hundreds of thousands of “tire miles,” because most tires survive well past the 45,000 mile point.