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The phrase Xkm is plural in every instance, unless the value of X is exactly 1 (assuming km is the head noun in the noun phrase). We can see this from the fact that in speech, the word kilometres (or kilometers for American readers) has the plural suffix, 'S', apart from when preceded by the numeral 1:

• 1 kilometre
• 0.5 kilometres
• 1.5 kilometres
• 1,000 kilomtres

However, when such phrases are the subjects of sentences, we see both singular and plural verb agreement (arguably singular is more frequent). This is because we can conceive of 2km as a single distance, or as two individual kilometres. As is the case, for example, with collective nouns, it is how we perceive the subject that matters, not its grammatical number or its plural or singular morphology.

This is nothing special about kilometre noun phrases. We see this type of singular agreement with all types of plural measure phrases:

• Twenty kilos is quite a lot to have to carry around all day.
• Two tonnes is the absolute limit.
• 100 decibels is far too loud.
• Three up-votes is not nearly enough.

Here is an Ngram which might give us a vague idea of the relative frequency of kilometres is and kilometres are. The blue line is kilometres is, the red kilometres are:

The phrase Xkm is plural in every instance, unless the value of X is exactly 1 (assuming km is the head noun in the noun phrase). We can see this from the fact that in speech, the word kilometres (or kilometers for American readers) has the plural suffix, 'S', apart from when preceded by the numeral 1:

• 1 kilometre
• 0.5 kilometres
• 1.5 kilometres
• 1,000 kilomtres

However, when such phrases are the subjects of sentences, we see both singular and plural verb agreement (arguably singular is more frequent). This is because we can conceive of 2km as a single distance, or as two individual kilometres. As is the case, for example, with collective nouns, it is how we perceive the subject that matters, not its grammatical number or its plural or singular morphology.

This is nothing special about kilometre noun phrases. We see this type of singular agreement with all types of plural measure phrases:

• Twenty kilos is quite a lot to have to carry around all day.
• Two tonnes is the absolute limit.
• 100 decibels is far too loud.
• Ten up-votes is not nearly enough.

Here is an Ngram which might give us a vague idea of the relative frequency of kilometres is and kilometres are. The blue line is kilometres is, the red kilometres are:

The phrase Xkm is plural in every instance, unless the value of X is exactly 1 (assuming km is the head noun in the noun phrase). We can see this from the fact that in speech, the word kilometres (or kilometers for American readers) has the plural suffix, 'S', apart from when preceded by the numeral 1:

• 1 kilometre
• 0.5 kilometres
• 1.5 kilometres
• 1,000 kilomtres

However, when such phrases are the subjects of sentences, we see both singular and plural verb agreement (arguably singular is more frequent). This is because we can conceive of 2km as a single distance, or as two individual kilometres. As is the case, for example, with collective nouns, it is how we perceive the subject that matters, not its grammatical number or its plural or singular morphology.

This is nothing special about kilometre noun phrases. We see this type of singular agreement with all types of plural measure phrases:

• Twenty kilos is quite a lot to have to carry around all day.
• Two tonnes is the absolute limit.
• 100 decibels is far too loud.
• Three up-votes is not nearly enough.

Here is an Ngram which might give us an idea of the relative frequency of kilometres is and kilometres are. The blue line is kilometres is, the red kilometres are:

The phrase Xkm is plural in every instance, unless the value of X is exactly 1 (assuming km is the head noun in the noun phrase). We can see this from the fact that in speech, the word kilometres (or kilometers for American readers) has the plural suffix, 'S', apart from when preceded by the numeral 1:

• 1 kilometre
• 0.5 kilometres
• 1.5 kilometres
• 1,000 kilomtres

However, when such phrases are the subjects of sentences, we see both singular and plural verb agreement (arguably singular is more frequent). This is because we can conceive of 2km as a single distance, or as two individual kilometres. As is the case, for example, with collective nouns, it is how we perceive the subject that matters, not its grammatical number or its plural or singular morphology.

This is nothing special about kilometre noun phrases. We see this type of singular agreement with all types of plural measure phrases:

• Twenty kilos is quite a lot to have to carry around all day.
• Two tonnes is the absolute limit.
• 100 decibels is far too loud.
• Three up-votes is not nearly enough.

Here is an Ngram which might give us a vague idea of the relative frequency of kilometres is and kilometres are. The blue line is kilometres is, the red kilometres are:

The phrase Xkm is plural in every instance, unless the value of X is exactly 1 (assuming km is the head noun in the noun phrase). We can see this from the fact that in speech, the word kilometres (or kilometers for American readers) had the plural suffix, 'S' in cases apart from when preceded by the numeral 1:

• 1 kilometre
• 0.5 kilometres
• 1.5 kilometres
• 1,000 kilomtres

However, when such phrases are the subjects of sentences, we see both singular and plural verb agreement (arguably singular is more frequent). Is is because we can conceive of 2km as a single distance or as two individual kilometres. As is the case, for example, with collective nouns, it is how we perceive the subject that matters, not its grammatical number or its plural or singular morphology.

This is nothing special about kilometre noun phrases. We see this type of singular agreement with all types of plural measure phrases:

• Twenty kilos is quite a lot to have to carry around all day.
• Two tonnes is the absolute limit.
• 100 decibels is far too loud.
• Three up-votes is not nearly enough.

Here is an Ngram which might give us an idea of the relative frequency of kilometres is and kilometres are. The blue line is kilometres is, the red kilometres are:

The phrase Xkm is plural in every instance, unless the value of X is exactly 1 (assuming km is the head noun in the noun phrase). We can see this from the fact that in speech, the word kilometres (or kilometers for American readers) has the plural suffix, 'S', apart from when preceded by the numeral 1:

• 1 kilometre
• 0.5 kilometres
• 1.5 kilometres
• 1,000 kilomtres

However, when such phrases are the subjects of sentences, we see both singular and plural verb agreement (arguably singular is more frequent). This is because we can conceive of 2km as a single distance, or as two individual kilometres. As is the case, for example, with collective nouns, it is how we perceive the subject that matters, not its grammatical number or its plural or singular morphology.

This is nothing special about kilometre noun phrases. We see this type of singular agreement with all types of plural measure phrases:

• Twenty kilos is quite a lot to have to carry around all day.
• Two tonnes is the absolute limit.
• 100 decibels is far too loud.
• Three up-votes is not nearly enough.

Here is an Ngram which might give us an idea of the relative frequency of kilometres is and kilometres are. The blue line is kilometres is, the red kilometres are: