It has to happen at some point, not knowing the speed of the current. Finding the speed of the current is not that difficult and requires some simple math skills.
Using the speed/velocity formula tells us that speed/velocity is distance divided by time: V=D/T. This is used to determine the speed of the boat over a given distance and time. Lets say we travel one mile against the current and one mile with the current.
If it takes 5 minutes to travel 1 mile with the current we can figure we are going 10.4 knots.
Cell C2 will contain the formula =((A2/(B2/60))/1.15078
We divide B2 by 60 to convert the number to minutes otherwise it will calculate as hours.
We divide the base formula by 1.15078 to convert the numbers from MPH to Knots.
The same calculations will be done for going against the current. Foregoing repeating the same formula lets same it took 10 minutes to go 1 mile against the current leaving us with a speed of 5.2 knots.
The final formula will be to subtract the with current speed from the against current speed and divide by 2. Assuming the cells for the against the current will be directly under the previous formula it will look like this:
This gives us a current speed of 2.6 knots for the current.
Finding this speed can be very difficult when dealing with open ocean as there are numerous variables that effect the current speed. Thankfully, usually, the Coast Guard has readings on ocean currents along with chart plotters that, hopefully, have good data.