Short answer: You need the substance’s density. Multiply volume in liters by density in kg/L to get mass in kilograms.
Explanation
Mass and volume are related by density. The basic formula is:
$m=\rho\times V$
where $m$ is mass, $\rho$ is density, and $V$ is volume.
- If density is given in kg/L:
$m\;(\text{kg})=V\;(\text{L})\times\rho\;(\text{kg/L})$.
- If density is given in g/mL (same as g/cm^3): note that $1\text{ g/mL}=1\text{ kg/L}$, so you can use the number directly:
$m\;(\text{kg})=V\;(\text{L})\times\rho\;(\text{g/mL})$.
- If density is given in kg/m^3, use that $1\text{ L}=0.001\text{ m}^3$, so:
$m\;(\text{kg})=V\;(\text{L})\times\frac{\rho\;(\text{kg/m}^3)}{1000}$.
Quick examples
- Water (approx. $\rho=1\text{ kg/L}$):
$V=2.5\text{ L}\Rightarrow m=2.5\times1=2.5\text{ kg}$.
- Olive oil (typical $\rho\approx0.92\text{ kg/L}$):
$V=3\text{ L}\Rightarrow m=3\times0.92=2.76\text{ kg}$.
- Using kg/m^3 (air ~1.225 kg/m^3):
$V=1\text{ L}\Rightarrow m=1\times\frac{1.225}{1000}=0.001225\text{ kg}$.
Practical notes
- For many liquids (water-based) you can approximate $1\text{ L}\approx1\text{ kg}$, but this is only exact for pure water at ~4°C; other liquids differ.
- Temperature and purity change density slightly; for precise work look up the density at the relevant temperature.
- For powders/solids measured in liters (e.g., sand, flour), use bulk density (not solid density) because packing/air gaps matter.
If you tell me the liquid (or its density), I can convert any given liters to kilograms for you.