Сумму к возврату считают исходя из налоговой базы.

Because it is the direct object of считают (they calculate / one calculates).

The dictionary form is сумма. In the accusative singular, it becomes сумму.

So the structure is basically:

• считают что? → сумму
• calculate what? → the amount

Since this is about money, сумма here is best understood as amount rather than the mathematical sum.

Literally, к возврату means something like for return or for refund.

• к normally means to / toward, but in official or bureaucratic language it can also mean for in the sense of intended for or due for
• возврату is the dative singular of возврат (return, refund)

So сумма к возврату is a set phrase meaning:

• the amount to be refunded
• the refundable amount
• the amount due back

This is common in formal financial language.

Because the preposition к normally requires the dative.

So:

• возврат → dictionary form
• к возврату → for refund / due for return

This is not the most literal or everyday way to say it in English, but in Russian official style it is very normal.

This is an indefinite-personal construction.

Russian often uses 3rd person plural with no stated subject when the doer is unspecified, similar to:

• they calculate
• people calculate
• it is calculated
• one calculates

So считают does not necessarily mean that several specific people are calculating it. It often just means that this is how it is done in general.

In smoother English, you would often translate the whole thing passively:

• The refundable amount is calculated based on the tax base

Because the sentence describes a general rule, procedure, or method, not one completed calculation.

• считать = imperfective, to calculate / to count
• посчитать / рассчитать = perfective forms, more like to calculate once / to complete the calculation

In instructions, regulations, and explanations, Russian often uses the imperfective present to describe what is normally done:

• Сумму к возврату считают...
• They calculate the refund amount... / The refund amount is calculated...

So this is about the process in general.

Исходя из means based on, proceeding from, or using ... as the basis.

It comes historically from the verbal adverb исходя from исходить, but in modern Russian it is very often treated almost like a fixed expression in formal language.

So:

• исходя из налоговой базы = based on the tax base

This expression is especially common in official, legal, and bureaucratic Russian.

Because из requires the genitive.

The phrase is:

• исходя из чего? → из налоговой базы

Forms:

• налоговая база = nominative singular
• налоговой базы = genitive singular

Both words change because the adjective must agree with the noun:

• налоговая → налоговой
• база → базы

Historically, исходя is a verbal adverb, but for learners it is often easiest to treat исходя из as a fixed formal expression meaning based on.

That is the safest practical approach.

A more grammar-heavy explanation is:

• исходя comes from исходить
• together with из, it forms an adverbial expression
• in modern official Russian, it behaves very much like a set phrase

So in this sentence, you do not need to analyze it as an action like while proceeding from. Just read it as based on.

Russian word order is flexible, and this order puts the focus first on what is being calculated: the amount to be refunded.

A more neutral order could be:

• Считают сумму к возврату исходя из налоговой базы

But the original order sounds natural in formal or explanatory writing because it introduces the topic first:

• As for the refundable amount, it is calculated...

So the fronting of Сумму к возврату is not strange; it is a normal way to organize information.

Usually no in a sentence like this.

Here, исходя из налоговой базы is tightly connected to the verb считают and tells you how / on what basis the amount is calculated.

So the normal punctuation is:

• Сумму к возврату считают исходя из налоговой базы.

A comma may appear in other contexts depending on sentence structure and emphasis, but in this straightforward official sentence, no comma is expected.

It is definitely formal, especially bureaucratic / financial / tax-related Russian.

Clues include:

• сумма к возврату
• исходя из
• налоговая база

In everyday conversation, people might say something simpler. But in documents, websites, tax instructions, and official explanations, this style is very common.

Yes, in technical or legal English, tax base is a correct translation.

For learners, it helps to know that база here does not mean a physical base. It means the basis for calculation, the amount or figure on which tax is calculated.

So:

• налоговая база = tax base / taxable base

In this sentence, it means that the refund amount is worked out using that figure as the basis.

Yes. A few possible rephrasings are:

• Сумму возврата рассчитывают на основе налоговой базы.
• Сумма к возврату рассчитывается исходя из налоговой базы.
• Возвращаемую сумму рассчитывают исходя из налоговой базы.

These versions may feel clearer to a learner because:

• рассчитывают is more explicitly calculate
• на основе is often easier than исходя из
• рассчитывается gives a passive-like meaning directly

But the original sentence is still perfectly normal in official Russian.