Wayli learning tool · Mortgage & investing

Mortgage & Investing Calculator

Explore questions about overpaying your mortgage, investing spare cash and comparing the two. Choose a question, enter your figures and get a structured summary you can take into your preferred AI to discuss your options further.
Why use Wayli before AI?

AI can explore your options, but it needs the right facts and assumptions first. Wayli runs the comparison, explains what drives the result and creates a structured summary you can use to continue the conversation.

Built to help my family. Shared in case it helps yours.

Why this tool exists

This tool was built to help my family explore whether spare money might be better used to overpay a mortgage or invest instead.

We are sharing the comparison in case it helps you understand the same trade-off. It shows what happens under the figures and assumptions entered. It does not tell you what to do.

Choose a question

Each question uses the same mortgage and investment model, but brings a different part of the result to the front. Choose the question closest to what you are trying to understand, then enter your figures below.

Your comparison

Enter your figures

Enter your mortgage details, monthly spare cash and an assumed investment return to see how the two paths compare.

Monthly payment is required.

Before relying on this scenario

  • Lender overpayment limits and early repayment charges are not modelled.
Assumptions and limitations

This Wayli tool helps you explore a current picture based on the information you've entered and the assumptions below. It does not tell you what to do, but it will help you understand your options, the trade-offs involved, and the factors most likely to change the outcome.


What we assumed

  • Investment returns are assumed to average the annual rate you've entered over the comparison period.
  • Investment growth is compounded monthly.
  • Monthly investments continue for the full comparison period.
  • Mortgage interest savings and investment growth are compared over the same timeframe.

What we left out

  • Personal tax calculations.
  • Investment fees and platform charges.
  • Future changes to mortgage interest rates.
  • Future changes to investment returns.
  • Inflation.
  • Personal preferences, such as the value you place on becoming mortgage-free.

What could change the result

  • Changing the expected investment return.
  • Changes to your mortgage interest rate.
  • Changing how much you invest or overpay each month.
  • Changing the comparison period.
  • How much you value the certainty of becoming mortgage-free.
Related questions

Continue Exploring

Continue with another decision or examine the research and experiments behind Wayli.