There’s not a separate savings credit calculator as such (as far as I can see) it’s taken into account as part of the Pension Credit Calculator.
As long as you’re 65 and the income is at least £103.15 for a single person or £164.55 per week, you may be entitled to the Savings Credit. There are examples on the Pension Service site but it doesn’t explain how the Savings Credit figure is arrived at but I'll have a go for you.
One of the examples is a couple whose income is £218.90 per week who qulify for Savings Credit of £23.41 per week. They have £12,000 in savings and that counts as £4 a week (because they say it’s £1 for every £500 above £10,000) and the other £214.90 is from state pensions and a personal pension.
The most Guaranteed Pension Credit for a couple is £209.70 per week and if the income exceeds that, the Savings Credit is reduced by 40% of the difference between the income and the £209.70.
In this example, the couple has an income that is £9.20 more than the maximum amount of Guaranteed Pension Credit and 40% of that is £3.68. Take that off the maximum Savings Credit of £27.09 for a couple and that leaves them with £23.41 a week. The maximum Savings Credit for a single person is £20.52 a week