I started with a 2-inch foam topper from a local shop. In my past, I was sure you needed at least 4 inches for any real comfort. That idea changed after a month on the thinner one. It added just enough softness to my too-firm mattress without the stuck-in feeling. Your mileage may vary, but for my back, the 2-inch was the sweet spot. Now I suggest trying a thinner layer first, as it can be enough without the cost.