to overturn / estimate
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to overturn
To roll over several times around a vehicle's longitudinal or transverse axis, usually in an accident.
Das Auto kam von der Straße ab und hat sich mehrfach überschlagen.
The car veered off the road and overturned several times.
'Überschlagen' is an inseparable verb. The prefix 'über-' stays firmly attached to the verb 'schlagen' in all sentence forms and tenses and is never placed at the end of the sentence.
Ich überschlage nur schnell die Rechnung.
I'm just quickly estimating the bill.
Pay attention to whether the verb is used reflexively ('sich überschlagen') or non-reflexively. Used reflexively, it often describes uncontrolled events; used non-reflexively, it describes a deliberate action such as estimating or skipping.
Wegen des Glatteises hat sich der LKW überschlagen.
Because of the black ice, the truck overturned.
With the reflexive form 'sich überschlagen', the subject reveals the exact meaning. Is it a vehicle (accident), 'die Ereignisse' (events; rapid sequence), or 'die Stimme' (voice; emotion)?
Das Rennauto hat sich in der Kurve überschlagen.
The race car overturned on the bend.
In the meaning 'to calculate roughly', 'überschlagen' is typically used with financial or numerical information. Common objects are 'die Kosten' (the costs), 'die Preise' (the prices), 'die Summe' (the total), or 'die Zahlen' (the numbers).
Ich habe die Reisekosten nur grob überschlagen.
I only roughly estimated the travel costs.
When 'überschlagen' means 'to skip', it usually refers to parts of a text. Typical objects are 'eine Seite' (a page), 'ein Kapitel' (a chapter), 'einen Absatz' (a paragraph), or 'eine Zeile' (a line).
Pass auf, dass du beim Kopieren keine Zeile überschlägst.
Be careful not to skip any line while copying.
The expression 'die Beine überschlagen' (to cross one's legs) is a fixed combination. The object here is always 'die Beine' in the accusative.
Er lehnte sich zurück und überschlug die Beine.
He leaned back and crossed his legs.
to estimate / appreciate
'schätzen' and 'überschlagen' can both mean that you do not have an exact number, but only calculate or guess approximately. 'schätzen' is more general: you can estimate an amount, a price, or even an age. It can also simply mean that you guess something. 'überschlagen' is used mainly when you calculate quickly and roughly, for example costs or a total. Learners may confuse the words because both are inexact. The simple difference is: 'schätzen' is an approximate statement; 'überschlagen' is a quick rough calculation.
Vor dem Einkauf überschlage ich kurz die Kosten.
Before shopping, I quickly estimate the costs.
Ich schätze, dass etwa dreißig Leute zur Feier kommen.
I estimate that about thirty people are coming to the party.