In my case it was the first year of receiving OAP and there is no end of year statement saying how much I received during the year so I reported how much went into my bank account. I apparently got it wrong as I thought the payment I received on April 19th was to be reported in the next tax year.
The yearly amount to report is a great mystery for most people.
Near the end of March each year you will receive a letter from the Pension Service giving the weekly amount you will receive each week for the State Pension for the coming year. Generally, if you multiply by 52, you can declare the result as the amount of State Pension received for that coming year when completing the self assessment for that year. For me, this year (20/21), HMRC have agreed.
I say generally, because in the past it hasn't always worked. The letter will give the date when the new amount will commence, and a similar letter for the following year will tell you when the year ends (by default) and when the next year commences. You'll often find it's not an exact 52 weeks within the April 6 to April 5 dates, and therein lies the rub. Pendants may want to work out the amount by calculating the exact number of days (which still doesn't always work). HMRC may, or may not, agree.
Don't worry about it. When you receive the
final self assessment calculation (I've had 3 this year) HMRC will include the amount in UK source income it feels you owe. If it's close to the amount you estimate, it's simplest to just agree. Not the route for pendants, but the one I use.
How much went into your bank account during the tax year very rarely gives the right amount for HMRC.