A jobless couple with two children will be Â£221 short of what would be needed for an acceptable standard of living every week by 2020, according to the Joseph Rowntree Foundation (JRF).
Pensioners, in contrast, will have Â£15 more a week than the minimum level.
The calculations are based on JRF's estimate of an acceptable income level.
Research for the Foundation suggested that families with two parents in full-time employment, workers without children, and pensioners would typically become better off over the next five years owing to the changes made in Chancellor George Osborne's Budget in July.
Lone parents and families with more than two children would see their living standards stagnate or fall, it claimed.
Major Budget announcements including the introduction of a National Living Wage and cuts to benefits such as tax credits would affect people's finances over the coming years, it said.
"The summer Budget has transformed the relationship between pay, benefits and work incentives. The National Living Wage is a game changer for some on low incomes as the new, higher rate will make work pay for more people," said Julia Unwin, the JRF's chief executive.
"But the wage rise comes hand in hand with changes to in and out of work benefits. Families will only be able to make ends meet if they have two parents in full-time work, but those who are able to find extra work will face a difficult juggling act as they try and make longer hours fit around family life.
"Lone parents, even those working full time, and people who are searching for work face a decade of sharply declining living standards."
Immediately after the Budget, the independent Institute for Fiscal Studies (IFS) said that 13 million UK families would lose Â£260 a year on average owing to the Budget's tax and benefits changes.
Out-of-work couples with children face "a decade of sharply declining living standards" owing to changes made to benefits, anti-poverty campaigners say.