The party said the cash would ensure spending was protected “from cradle to college” and went beyond other parties’ commitments on education.
Leader Nick Clegg will say the plans are all about boosting opportunity.
But the Conservatives said the Lib Dems offered “uncertainty for parents” while Labour said Nick Clegg’s party had “broken their promises” in government reports BBC.
The Lib Dems are the last of the three largest Westminster parties to launch their manifestos after Labour and the Conservatives.
The BBC’s assistant political editor Norman Smith said he expected it to be a “minimalist, pared-back” document, with a focus on a few key priorities, after the party was unable to deliver its main commitment on tuition fees from 2010.
The Liberal Democrats are trying to stake out a claim to be the party that makes education a spending priority, by the promise of an extra £2.5bn.
Their education-friendly image had taken some hard knocks from the tuition fee U-turn and being in a coalition government that frequently clashed with the teachers’ unions.
But they have put forward a spending plan which they hope will out-flank both the Conservatives and Labour.
Labour pledged to protect school budgets against inflation, while the Conservatives’ offer was to protect per-pupil spending at a time of rising pupil numbers.
The Liberal Democrats’ pitch is to combine both – promising to protect per-pupil spending in real terms , including for an extra 460,000 pupils.
But there is a tough warning from the Institute for Fiscal Studies that school costs are rising much faster than inflation and a looming school funding shortage will face whoever wins the election.