reckon with
To consider as one of the things which may change a situation; consider (something) that will make a difference in the results.
The coach said the opposing pitcher had a fast ball to be reckoned with.
reckon without
To fail to consider as one of the things which might change a situation; not think about.
The committee for the class picnic party made careful plans for a beach party but they reckoned without a sudden change in the weather.