An estimate
An estimate is a calculated guide based on the information you enter. Nothing has been verified and no lender has looked at it.
Its job is to tell you roughly where you stand and what is helping or hurting your position, so you can decide whether it is worth taking the next step.
A pre-approval
A pre-approval is a conditional indication from a lender that they would be willing to lend up to a certain amount, subject to conditions.
It usually runs for a limited time and can carry conditions such as a satisfactory property, confirmation of your deposit or a valuation. It gives you real confidence when making an offer, but it is not the final word.
A formal approval
Formal approval comes after a lender has verified your income, identity and deposit, and has assessed the specific property you are buying.
This is the point at which the lending is genuinely committed, and it is tied to that property rather than to you in general.
Who makes the final call
No calculator can approve lending. A lender applies its own policy, its own testing and its own view of your documents, and its decision is the one that counts.
That is not a reason to skip the estimate. It is the fastest way to find out whether a conversation with a lender or adviser is worth having now or in a few months.
Where to from here
Start with the estimate, then take it to someone who can turn it into a pre-approval when the numbers look workable.