Labour swiftly dismissed the number as "made up".
LibDem sources explained that the analysis was based on Ed Balls' announcement that Labour would clear the deficit on the current day-to-day spending budget and put national debt on a downward path by 2020.
He has not said his party would clear the overall deficit, leaving scope to borrow and spend more on housing and other projects.
The calculations include Labour's pledge to match the Coalition's plans for 2015/16 but have not included any extra investment.
Under Coalition plans, public sector net borrowing goes from £99bn this year to zero by 2019/20.
But they say if Labour won, £41bn would still be borrowed in 2019/20 and £32.6bn in 2020/21.
The cumulative total of the extra borrowing under Labour is worked out at £166bn.
" Danny Alexander, chief secretary to the Treasury said: "Their new borrowing bombshell will pile another £166bn onto the debt mountain left by their catastrophic mismanagement of the UK economy."