A simple and generic question that I can not find a straight answer for. The rule that I am debating is based on that I should write 2 if it is a measured value. In this situation the 2 is a rounded measured value from 1.96 which would make me think to use two now instead of 2. What is technically correct and is the technical answer best for a user experience?

When to use TWO and When to use 2?

Each Electric battery provides Two kWh of plug-in energy storage for $700.


Each Electric battery provides 2 kWh of plug-in energy storage for $700

  • As the answer states, this isn't usually a UX centric issue as much as it is a content style issue. The Chicago Manual is but one of many style guides (including writing your own) that your company may want to use as reference.
    – DA01
    Jun 11, 2015 at 1:00
  • @DA01 After reading your comment it just triggered a thought, that content plays very important role in delivering the best UX. It is the core around which all the interactions are designed. But it demands a different set of skill. Perhaps that's why we have a different team focusing on everything related to content. Style, grammer, chunking, etc. Just a thought.
    – Praasshant
    Jun 11, 2015 at 4:17
  • @pzw I agree completely. Content is a primary aspect of UX. So I don't think it's necessarily off-topic in general. To spin this question towards UX, one could argue the key isn't so much two vs. 2 but rather that you simply pick one and be consistent with it...which is what a content style guide can help with.
    – DA01
    Jun 11, 2015 at 11:42

3 Answers 3


The Chicago Manual of Style and AP have detailed information regarding this. In general if it is part of a paragraph and the intended reader is getting textual information then it ought to be spelled out (two as opposed to "2").

If the text is a list of accounting or technical information then numbers are appropriate.

If it's something in between the two extremes then decide on a format and be consistent.

The Chicago Manual of Style

Starting with the fundamentals, Chicago says when a number begins a sentence, it is always spelled out regardless of complexity.

If this number is followed by another number of the same category then only the first number should be spelled out.

Take for instance the following example: "One hundred twenty-five of the 200 surveyed websites ..."

The more this concerns you the more you need to consult existing style manuals. The Chicago and AP Manuals are authoritative in the US. (Academics often follow the MLA.)

If your audience is European then ... [I just did a quick search] ... A good starting off point is: http://en.wikipedia.org/wiki/Style_guide


Agree regarding use of a style manual to be consistent with formal convention. You may also want to benchmark against any industry leader to check consistency with what consumers are used to seeing (popular convention).

In some cases formal and popular conventions do not match. If this is the case and popular usage is inconsistent then congratulations, you get to be a trailblazer. Follow others advice here regarding consistency in this case.

Consistency is important for clarity of meaning as well as cognitive debt avoidance (don't make me think).


Original Question: "When to use TWO and When to use 2? Each Electric battery provides Two kWh of plug-in energy storage for $700. or Each Electric battery provides 2 kWh of plug-in energy storage for $700"

Your question falls within Engineering or Technical writing and not Chicago or AP style. When referencing Technical writing, as in a formula, always use the number not the word. kWh is a formula (1000 Watts per hour over a 24 hour period) Here are the examples:

"two one thousand watts per hour over a 24 hour period Battery" Not Technical "2 kWh Battery" Correctly written formula. "It takes two 2 kWh batteries for a system" Correctly written sentence.

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