File: (L-R) Students Noah Zambrana, 5, and Elda Gelagay, 5, show their work to educator Mee Kim during a pattern game in a pre-kindergarten class at Campbell Elementary School in Arlington County. (Melina Mara/The Washington Post)

A proposed change in Virginia’s preschool funding formula could leave Arlington County with about 50 fewer state-funded slots, if the House of Delegates and Senate don’t agree on a fix in the coming days.

The House budget proposal would change the formula so that localities must calculate the number of eligible students for the publicly funded pre-kindergarten program for at-risk children using projected kindergarten enrollments.

In the past, they were able to estimate enrollment using Virginia Employment Commission figures as a baseline for determining the number of eligible students living in poverty, which often yielded higher numbers.

By requiring every district to use the kindergarten enrollment figures instead, 32 localities would lose nearly 900 pre-kindergarten slots statewide, according to a calculation from Voices for Virginia’s Children, an advocacy group.

The senate already passed a budget amendment that would hold districts harmless if they would lose slots through the new formula. The amendment would also increase slightly - by $98 dollars - the per pupil allocation to account for inflation, something that has not been done since 2008.

Now that the two chambers have passed different budget plans they go through a process to reconcile them.