The way to examine your rental yield is as a percentage, using the following equation:
Yearly rental income / total investment * 100 = gross rental yield
£6000 (yearly rental income) / £100,000 (total investment) * 100 = 6% (gross rental yield)
Remember, your total investment should include how much it has cost you to buy the property including legal fees, refurbishment costs, your mortgage and everything else that you have paid out for in the property purchase.
To help you on your way we have created a rental yield calculator for your use.