Which everyday calculation should we add next?

I’ve been building Utility Tools — a collection of free online calculators and small utility tools for everyday tasks.

We currently have 15 tools, including percentage, discount, tip, age, BMI, EMI, GST, unit conversion, word count and more.

I’m trying to decide what would actually be useful to add next rather than just adding tools for the sake of having more.

What calculation or small utility do you find yourself searching for repeatedly?

If you have a specific use case, I’d especially like to hear it.