jq not working with variables, throws error "jq: error: $var is not defined at "

Question

let var=5*86400
echo $var

Output - 432000

Trying to execute this variable with jq command

jq --null-input '$var'

Output -

jq: error: $var is not defined at <top-level>, line 1:
$var
jq: 1 compile error

jq works fine without any variable

jq --null-input '5*86400'

Output - 432000

Can anyone help me how to use a variable with jq query?