Resto vs. remanente
In Spanish, both resto and remanente translate to the English concepts of rest or remainder, but their usage depends largely on the context and the level of formality. While resto is an everyday word used in a wide variety of casual situations, remanente is a formal term primarily reserved for financial, administrative, or technical contexts.
Resto
The word resto is the most common and versatile way to say rest or remainder in everyday Spanish. It is used to refer to people, time, food, money, or everyday objects that are left over or remaining. When used in its plural form, restos, it often translates to leftovers, ruins, or remains.
Me comeré el resto de la pizza mañana por la mañana.
(I will eat the rest of the pizza tomorrow morning.)
El resto de los invitados llegará después de la cena.
(The rest of the guests will arrive after dinner.)
Pasaremos el resto del día descansando en la playa.
(We will spend the rest of the day resting at the beach.)
Guarda el resto del dinero en tu cartera para emergencias.
(Keep the rest of the money in your wallet for emergencies.)
Los restos del castillo medieval son una gran atracción turística.
(The remains of the medieval castle are a great tourist attraction.)
Le di el resto de mi almuerzo a los pájaros del parque.
(I gave the rest of my lunch to the birds in the park.)
Remanente
The word remanente is a more formal and technical term that translates to remainder, surplus, or balance. It is almost exclusively used in financial, legal, scientific, or administrative contexts to describe the exact amount or quantity left over after a specific calculation, process, or distribution.
La empresa invertirá el remanente de sus ganancias en nuevos equipos.
(The company will invest the remainder of its profits in new equipment.)
El juez ordenó distribuir el remanente de la herencia entre los nietos.
(The judge ordered to distribute the remainder of the inheritance among the grandchildren.)
Existe un remanente de vacunas que debe ser utilizado esta misma semana.
(There is a surplus of vaccines that must be used this very week.)
El presupuesto del año pasado dejó un remanente de un millón de dólares.
(Last year's budget left a balance of one million dollars.)
Los científicos midieron cuidadosamente la energía remanente en el reactor.
(The scientists carefully measured the remaining energy in the reactor.)
El gobierno municipal utilizará el remanente fiscal para reparar las calles.
(The municipal government will use the fiscal surplus to repair the streets.)
Summary
In short, you should choose resto for everyday conversations about the remaining part of almost anything, such as time, food, or groups of people. You should reserve remanente for formal, financial, or technical situations where you are discussing a budget surplus, a calculated remainder, or an administrative balance.