And we took advantage of a healthy used-equipment market driving record retail sales to generate almost 30% more proceeds in the quarter than we did a year ago.
And I'm proud to report the team United delivered $873 million of adjusted EBITDA in the first quarter and they did it safely turning in another quarterly recordable rate below one.
With Franklin, we added 20 stores to our General Rental footprint in the Central and Southeast regions.
And we're continuing to invest in growing our specialty network with six cold starts year-to-date and another 24 planned this year.
Customer sentiment continues to trend up in our surveys, as a majority of our customers expect to see growth over the next 12 months.
Rental revenue for the first quarter was $1.67 billion, which was lower by $116 million or 6.5% year-over-year.
Within rental revenue, OER decreased $117 million or 7.7%.
In that a 5.7% decline in the average size of the fleet was a $87 million headwind to revenue.
Inflation of 1.5% cost us another $24 million and fleet productivity was down 50 basis points or a $6 million impact.
Sequentially fleet productivity improved by a healthy 330 basis points recovering a bit faster than we expected.
Finishing the bridge on rental revenue this quarter is $1 million in higher ancillary and rerent revenues.
As I mentioned earlier, used equipment sales were stronger than expected in the quarter, coming in at $267 million, that's an increase of $59 million or about 28% year-over-year, led by a 49% increase in retail sales.
The end market for used equipment remained strong and while pricing was down year-over-year, it's up for the second straight quarter with margins solid at almost 43%.
Adjusted EBITDA for the quarter was just under $873 million, a decline of $42 million or 4.6% year-over-year.
The dollar change includes an $84 million decrease from rental, in that OER was down $86 million, while ancillary and rerent together were an offset of $2 million.
Used sales were tailwind to adjusted EBITDA of $19 million, which offset a $2 million headwind from other non-rental lines of business and SG&A was a benefit in the quarter up $25 million as similar sales the last couple of quarters.
Our adjusted EBITDA margin in the quarter was 42.4%, down 70 basis points year-over-year and flow through as reported was about 62%.
Adjusting for those two results is an implied detrimental flow through for the quarter of about 37%.
Our shift to adjusted EPS, which was $3.45, that's up $0.10 versus Q1 last year, primarily on lower interest expense and a lower share count.
For the quarter gross rental capex was $295 million.
Our proceeds from used equipment sales were $267 million, resulting a net capex in Q1 of $28 million.
Now turning to ROIC, which remained strong at 8.9%.
As we look back over what's obviously been a challenging 12 months.
Free cash flow was also strong at $725 million for the quarter.
This represents an increase of $119 million versus the first quarter of 2020 or about a 20% increase.
As we look at the balance sheet, net debt is down 21% year-over-year without having reduced our balance by about $2.3 billion over those 12 months.
Leverage continues to move down and was 3 times at the end of the first quarter, that compares with 2.5 times at the end of the first quarter last year.
We finished the quarter with over $3.7 billion in total liquidity that's made up of ABL capacity of just under $3.2 billion and availability on our AR facility of $276 million.
We also had $278 million in cash.
And third, the contribution of our Franklin Equipment acquisition, which we estimate at about $90 million of revenue and $30 million of adjusted EBITDA for the remainder of the year.
Bonus expense remains the headwind we've discussed previously, and at midpoint is about a 60 basis point drag in margin year-over-year.
Finally, the increase in free cash flow reflects the puts and takes, from the changes I mentioned and remains robust at a midpoint of $1.8 billion.

Summary:
Our shift to adjusted EPS, which was $3.45, that's up $0.10 versus Q1 last year, primarily on lower interest expense and a lower share count.