What is the basic Roman unit of length?

The basic Roman unit of length was the pes, or Roman foot. Modern reconstructions place it at about 296 millimeters. Larger units such as the passus (pace) and the Roman mile were defined in terms of the Roman foot.

How long is a Roman mile in modern units?

A Roman mile, mille passus, was defined as one thousand paces or five thousand Roman feet. Using a Roman foot of roughly 296 millimeters, a Roman mile corresponds to about 1.48 kilometers.

What was the Roman unit of volume for liquids?

The principal Roman liquid volume was the amphora quadrantal, equal to one cubic Roman foot and probably around 26 litres. Smaller units, such as the congius and the sextarius, were defined as fixed fractions of the amphora.

How heavy was a Roman pound (libra)?

The Roman pound, or libra, is estimated at roughly 329 grams. It was divided into 12 unciae or ounces. Many later European weight systems were historically related to this libra unit.

Are historical Roman unit values exact?

Original Roman standards varied slightly by time and place and have to be reconstructed from archaeological and textual evidence. The values used in this calculator follow widely accepted modern estimates and are suitable for study, teaching and general reference.

Roman Units Converter

Roman numerals & ancient measures

Convert Roman numerals to decimal numbers and back, and convert ancient Roman units of length, volume and weight (pes, passus, mille passus, amphora, sextarius, libra, uncia…) to modern SI units.

Note: All Roman measures are based on widely accepted modern reconstructions (Roman foot ≈ 296 mm, amphora ≈ 26 L, libra ≈ 329 g). Ideal for teaching, historical research and hobby projects.

Roman numeral converter (I–MMMCMXCIX)

Convert between modern decimal numbers and classical Roman numerals using standard subtractive notation (IV, IX, XL, XC, CD, CM). The converter validates common mistakes.

Roman numeral

Valid symbols: I, V, X, L, C, D, M. Use uppercase or lowercase.

Decimal (Arabic) value

Decimal to Roman numerals

Decimal (1–3999)

Roman numeral

Roman length units → metres & kilometres

Based on a standard Roman foot (pes) of about 296 mm. Includes pes, uncia, cubitum, passus and the Roman mile (mille passus).

Value

Roman unit

Metres (m)

Kilometres (km)

Equivalent in pedes

Number of Roman feet corresponding to the input.

Presets:

Roman volume units → litres & m³

Uses the amphora quadrantal as one cubic Roman foot, roughly 26 litres. Sextarius and congius are fixed fractions of the amphora.

Value

Roman unit

Litres (L)

Cubic metres (m³)

Equivalent in sextarii

Presets:

Roman weight units → grams & kilograms

Based on a Roman pound (libra) of about 329 grams, subdivided into 12 unciae (ounces).

Value

Roman unit

Grams (g)

Kilograms (kg)

Equivalent in librae

Presets:

Roman numerals vs. Roman units – two different systems

Modern websites often focus either on Roman numerals or on Roman measures. This page brings both worlds together: in the first tab you can convert numerals such as MMXXV to their decimal counterparts, while the second tab focuses on historical length, volume and weight units used throughout the Roman Empire.

Key assumptions behind the Roman units

Ancient standards were not perfectly uniform across provinces or periods. To make the converter practical and consistent, we adopt widely cited modern approximations:

Roman foot (pes): 0.296 m
Roman mile (mille passus): 1,000 paces = 1,480 m
Amphora quadrantal: about 26 L
Sextarius: 1/48 of an amphora (≈ 0.54 L)
Roman pound (libra): about 329 g

These values are close enough for courses in history, archaeology and engineering, and they keep internal ratios between Roman units exactly consistent.

How Roman numerals are interpreted

The numerals converter follows standard textbook rules:

Valid symbols: I, V, X, L, C, D, M.
Subtractive notation allowed only for I before V or X, X before L or C, C before D or M.
Repeated symbols limited to three in a row (e.g. III is valid, IIII is flagged).
Supported range: 1 to 3999 (I to MMMCMXCIX).

If you enter a malformed numeral, the tool will show an error message rather than silently returning an incorrect value.

FAQ: Roman units and this converter

Can I rely on these numbers in scientific work?

For high-level comparisons, teaching and exploratory calculations, yes. For original research on Roman metrology you should always quote the specific reconstruction (museum artefact, inscription or scholarly article) that motivates the values you use.

Why does the Roman mile differ from the modern mile?

The modern statute mile is about 1.609 km; the Roman mile is shorter at around 1.48 km because it is built from Roman feet, which are slightly shorter than modern imperial feet, and uses 1,000 paces as its definition.

How precise are the litre and gram conversions?

Internally the calculator keeps more decimal digits than it displays. The main source of uncertainty is historical, not numerical: different authors give slightly different reconstructions. Those differences are typically within a few percent.