In one sentence A litre and a millilitre measure the same thing — volume — and one litre contains exactly 1,000 millilitres.
The litre and the millilitre
Volume is the amount of space a liquid or a gas occupies. The two units a nurse meets most often are the litre, written L, and the millilitre, written mL. One litre contains exactly 1,000 millilitres, so they are not two different quantities; they are the same quantity written at two different scales. A large IV bag is labelled 1,000 mL, which is the same as 1 L. A medicine cup may be marked 5 mL or 10 mL, and a syringe may hold 2 mL or 20 mL. Because the metric system is built on tens, the prefix “milli-” always means one thousandth: a millilitre is one thousandth of a litre. Nothing is lost or gained in the change, because only the size of the written number changes, never the amount of fluid.
A 1,000 mL bag of saline is exactly 1 L of fluid. A 5 mL spoon of syrup is 0.005 L, a very small part of a litre.
An IV bag is labelled 500 mL. Is that more or less than half a litre, and how do you know?
Show the answer
It is exactly half a litre, because half of 1,000 mL is 500 mL. The bag contains 0.5 L, so it is neither more nor less than half a litre.
Read detailed explanations
- Density, the kilogram and why the litre is a derived unit
The rule that 1 L of water has a mass of about 1 kg is not a coincidence of the metric system; the units were designed together. Density is the mass of a fixed volume of a substance, written density = mass ÷ volume, and it is measured in grams per millilitre (g/mL) or kilograms per litre (kg/L). For pure water at about 4 °C the density is 0.99997 g/mL, which is why the working value 1 g/mL is used. The original metric definition went further: the kilogram was defined as the mass of one litre of water at its temperature of maximum density, and the litre was defined from a cube of water one tenth of a metre on each side — that is, 10 cm × 10 cm × 10 cm = 1,000 cm³, which is exactly 1,000 mL. Even though the modern kilogram is now fixed by a physical constant rather than by a water prototype, the numerical coincidence survives because the units were built to preserve it. Specific gravity, the ratio of a substance’s density to that of water, lets a nurse compare fluids quickly: a fluid with a specific gravity of 1.05 is 5 % heavier than water for the same volume. Clinical consequences follow. One litre of 0.9 % saline or 5 % glucose has a specific gravity close to 1.00, so it adds roughly 1 kg to a patient’s mass, whereas a litre of a lipid emulsion, whose droplets are less dense than water, adds slightly less. Reading a fluid balance chart against daily weights therefore only works when the fluid is water-based. One anomaly of water is worth a line: unlike nearly every other liquid, water expands when it freezes, so ice floats and 1 L of ice has a mass of only about 0.92 kg. A nurse who records 1 L of irrigation fluid returned from a patient as 1 kg is making an estimate, not a measurement, and the estimate should be labelled as such.
- Seconds, drip counting and pump programming
Seconds enter nursing arithmetic through the short observation windows used to verify a rate. A gravity infusion is checked by counting drops in a timed interval, most often 15, 30 or 60 seconds, and the count is then scaled up to a minute. A count taken over 15 seconds is multiplied by 4; a count taken over 30 seconds is multiplied by 2. That scaling is itself a time conversion, and it is where errors appear, because a nurse who counts for 20 seconds and multiplies by 3 has converted correctly only if 60 ÷ 20 = 3 exactly, and the arithmetic must be checked rather than assumed. Electronic infusion pumps remove the drop counting but not the time arithmetic. A volumetric pump is set in millilitres per hour and delivers a programmed volume over a programmed duration; a syringe driver is set in millilitres per hour, and some are set in millimetres per hour of plunger travel, which is not the same number and must never be transferred between devices. When a rate change is ordered, the nurse recalculates the remaining volume and the remaining time, then converts the new duration into the units the pump expects. Flushing a line, giving a bolus and documenting the exact time of a rate change all depend on reading a clock in hours and minutes while the pump logs in decimal hours and seconds. Two safety habits follow. First, programme the pump and then compare the displayed rate with an independent calculation, because a pump will deliver exactly what it was told, including a wrong decimal. Second, round to what the device can accept: a rate of 41.7 mL/hr may be settable only as 42 mL/hr on one pump and as 41.7 on another, and a drip count must be rounded to a whole drop because half a drop cannot fall. Finally, the 24-hour clock removes the a.m./p.m. ambiguity that produces wrong-hour errors, because 18:30 is unambiguous whereas “6:30” could be morning or evening, and a medication chart that loses twelve hours is a genuine hazard.
- The arithmetic of decimal hours, and how to check it
A decimal hour is a fraction of an hour, and the conversion back to minutes is a multiplication by 60. That single fact resolves every case. Ten minutes is 10 ÷ 60 = 0.1667 hours; fifteen minutes is 0.25; twenty minutes is 0.3333; thirty minutes is 0.5; forty-five minutes is 0.75. A nurse who memorises the quarter-hour family (0.25, 0.5, 0.75) and the twenty-minute family (0.333, 0.667) can read most ward records without a calculator. The reverse direction is the check: multiply the decimal by 60 and the minutes must return. If 1.4 hours is claimed to be 1 hour 40 minutes, the check is 0.4 × 60 = 24 minutes, so 1.4 hours is really 1 hour 24 minutes, and the claim is wrong by sixteen minutes. Rounding deserves its own rule because decimal hours are often carried to two places. Rounding 0.1667 to 0.17 is a rounding of about 12 seconds; rounding 0.3333 to 0.33 understates by about 12 seconds. Those differences are harmless on a monthly timesheet but not harmless when they accumulate across a shift of repeated short infusions, and they are dangerous when a rounding pushes a due time past a safety window. The professional habit is to keep the exact minutes in the clinical record and to round only in the summary line, stating the rounding. Estimation catches gross errors even earlier: one hour is 1 ÷ 24 ≈ 4.2 % of a day, so a four-hour infusion is about one sixth of a day, and any answer implying more than 24 hours in a day is impossible. Be alert, finally, to notation from software. Some hospital systems export durations as “1:30”, which means 1 hour 30 minutes, while others export “1.5”, which also means 1 hour 30 minutes; the same value wears two costumes, and the colon form must never be read as a decimal.