Litre L vs l vs ℓ vs 1: Which Symbol Is Correct?

The litre is unusual because it has two accepted symbols: lowercase l and uppercase L. The capital form was admitted for a practical reason—lowercase l can be almost indistinguishable from the digit 1 in some typefaces.

You may also encounter the script character ℓ. It has a historical association with litre notation, but it is not one of the two symbols listed in the current SI Brochure. In measurement notation, that makes it a look-alike worth recognizing rather than a third standard option.

Four similar shapes, only two accepted litre symbols

Why litre gets two accepted symbols

The BIPM lists the litre with the symbols L or l. Uppercase L is an explicit exception to the usual convention that unit symbols not derived from personal names begin with a lowercase letter.

The reason is practical: a simple lowercase l can look almost identical to the numeral 1. The CGPM adopted uppercase L as an additional symbol in 1979 to reduce that ambiguity. Both forms remain valid, so 1 L and 1 l can both represent one litre.

Where script ℓ fits

ℓ is U+2113 SCRIPT SMALL L. Unicode records two relevant associations: it is the mathematical symbol “ell,” and it has also been used as a traditional litre symbol. Unicode’s names list then points to ordinary l or L as the recommended symbols for litre.

That distinction matters in digital text. Using ℓ merely because it looks clearer creates a different Unicode character, and in mathematical material the same character may genuinely mean “ell” rather than a unit of volume. A global rule that converts every ℓ to L would therefore be unsafe.

Litre’s status in SI documents

The current SI Brochure lists the litre among non-SI units and gives the exact relation 1 l = 1 L = 1 dm³ = 10⁻³ m³. Its table therefore recognizes both letter forms while keeping the litre outside the SI itself.

For practical notation, the point is simple: use L or l for litre. The digit 1 is not a unit symbol, and ℓ is a separate Unicode character with other mathematical uses.

Why many writers prefer uppercase L

Both accepted forms are valid, but uppercase L is often clearer in labels, forms, user interfaces, OCR output, and mixed alphanumeric data. Compare 1 L with 1 l in a font where lowercase l is just a vertical stroke.

This is a legibility choice, not a claim that lowercase l is wrong. A publisher can use l consistently if its typography keeps the character distinct from 1.

Adding prefixes to litre

An SI prefix and a unit symbol form one inseparable token. Millilitre is therefore written mL or ml, depending on the accepted litre symbol, while the number remains outside the token: 250 mL.

Case still matters. Lowercase m is the milli prefix, whereas uppercase M is mega. The guide to m vs M in SI units explains why automatic capitalization can change a measurement by orders of magnitude.

For microlitres, digital text may contain more than one mu-shaped Unicode character. If exact encoding matters, the Micro Sign µ vs Greek Mu μ guide explains why two visually similar prefix characters can still have different code points.

What a validator should accept

When a field is known to contain volume units, a validator can safely accept L and l as litre symbols and reject the digit 1. It can also flag ℓ for review when the data standard expects the BIPM forms.

In unrestricted prose, however, L and l can be ordinary letters and ℓ can be a mathematical symbol. Context has to come before normalization. The safest workflow is to identify the field as a measurement expression, preserve the original characters, and only then apply unit-specific validation.

Choosing between L and l

Use L or l. Choose one consistently within a document or dataset, and prefer uppercase L when avoiding confusion with the digit 1 is important.

Do not substitute 1 for a litre symbol. Treat ℓ as a distinct legacy or mathematical character rather than as a decorative spelling of L.

Common prefixed forms

Once L or l has been chosen, ordinary SI prefixes can be attached to it. Examples include mL for millilitre, cL for centilitre, and dL for decilitre. The prefix is part of the unit symbol, so these forms are written without a space inside the symbol.

The readability advantage is clearest in short strings: 1 mL is harder to misread than 1 ml in a font where l resembles 1. This is one reason uppercase L is common in medical and technical material even though lowercase l remains accepted.

Microlitre introduces a separate character issue

For microlitre, the prefix is represented by a mu-shaped character followed by L or l. Real digital data may contain either U+00B5 MICRO SIGN or U+03BC GREEK SMALL LETTER MU, even when the rendered text looks the same.

That mu-encoding question is separate from choosing L or l. The related issue in µm and plain-text um notation shows why it is better to handle known variants explicitly than to flatten every similar-looking character into one bucket.