Your open-to-buy spreadsheet is probably right. Until it isn't.
We ran our open-to-buy math over a real buyer's workbook: about 14,800 rows, six sheets, three years. Every column reproduced to the cent. The 0.3% that did not match were cells where the workbook's own formulas had drifted, and none of them looked wrong on the screen.
The method is not the problem
Open to buy has been calculated the same way for decades, and a six-month merchandise plan built in Excel is a perfectly good way to do it. We can say that with more confidence than most, because we checked our own arithmetic against one rather than against a textbook.
Every column matched: the sales goal, the inventory plan, the forward plan, the open-to-buy itself, the averages underneath it. If your sheet is set up the way that one was, your numbers are right.
What actually goes wrong
Not the formulas. The document. A workbook is a living thing that gets copied between locations, dragged down a column, and inherited by whoever does the buying next. Here is what three years of that looked like in a real one.
- A tab cloned from another store. One location's sheet had been copied from another and its stock column never replaced. For a full year it reported the other store's inventory, and that figure fed the totals.
- Numbers that were a fill series. A column reading 84, 85, 86 in future months was not data. It was a cell dragged down, and it looked exactly like a forecast.
- References that slipped a row. A handful of cells pointed one row above where they should, pulling last month's figure into this month's plan.
- A #REF! in a totals cell, sitting there long enough that nobody saw it any more.
None of these raise an error. Every one of them produces a number that looks like all the other numbers in the column, which is why they survive for years. A spreadsheet cannot tell you that a figure is wrong. It can only tell you that a cell is empty.
Side by side, including where the sheet wins
| A spreadsheet | Rebridge | |
|---|---|---|
| The arithmetic | Correct. This is a solved problem and has been for decades. | The same arithmetic. We reproduced a real workbook to the cent. |
| Cost | Free, and you already own it. | Included with Rebridge, priced on purchase order volume. |
| Point of sale | Works with any of them, because you key the numbers in yourself. | Lightspeed Retail R-Series only today. |
| Getting the numbers in | By hand, every month, per category. | Synced. Sales, stock and open orders come from your POS. |
| On order, by month | Typed per month, per category, per brand. One workbook we saw carried 849 of those cells. | An order counts in the month it is due. One with no date counts in the current month. |
| Cost or retail | Whatever the author chose, usually undocumented, and nothing stops the two mixing. | A switch, and the two can never mix inside one calculation. |
| When a formula drifts | Nothing happens. It keeps printing a number. | There is no formula to drift, and a missing input shows as missing. |
| Changing how it works | Anything you want, immediately. This is a real advantage. | Growth and turn rate are yours; the method is fixed. |
The honest version
If you run one location, have an unusual method, and your sheet has not been copied or inherited, keep it. It is free, it does exactly what you tell it, and nothing here will serve you better.
The case for changing is the hours and the drift: a workbook copied between locations, passed between buyers, and fed by hand every month is where those faults come from, and no amount of care prevents them. If that is yours, the arithmetic was never the risk.
Questions
Is there anything wrong with running open to buy in a spreadsheet?
No, and the method is not in question. We ran our own open-to-buy math across about 14,800 rows of a real buyer's workbook, six sheets covering three years, and every column reproduced to the cent. The formulas were right. What went wrong was the document: references that slipped a row, a tab copied from another location that kept the original's stock figures, and a column of numbers that was a dragged fill series rather than data.
What exactly did you find in the workbook?
About 0.3% of cells disagreed with the formula the sheet itself was using elsewhere. Every one was damage rather than a difference of method: an off-by-one row reference, a #REF! left in a totals cell, rows that had shifted under a formula, and one year where a location tab cloned from another store still held the other store's stock column, which double-counted in the totals for twelve months.
Would I notice if my spreadsheet had that problem?
Probably not, and that is the point. None of those faults produce an error. They produce a number that looks like every other number in the column. A buyer would only catch the cloned tab by noticing that one location's stock figure looked like another's, twelve months after it started.
Does Rebridge use the same formula as my sheet?
Yes, including the part most published versions leave out. When stock falls below plan, plan becomes the closing basis rather than actual stock, which stops a category that is already short from reporting a larger open to buy than it should. Above plan the two versions agree exactly.
My sheet is at retail. Does that matter?
It matters a lot, and it is the most common silent error we see. Retail is the older convention and most six-month plans use it; cost is what you write on a purchase order. Both are correct. Mixing them inside one calculation is not, and a spreadsheet will happily let you subtract cost stock from a retail plan. Rebridge carries both as a switch and refuses to mix them.
How does on order work, compared with typing it in?
In a workbook, on order is entered per month, per category, per brand, from the purchase order folder. One sheet we looked at carried 849 of those cells. In Rebridge an open order counts in the month it is due to arrive and rolls into stock afterwards, so it is counted once. An order with no arrival date, or one whose date has passed while it is still open, counts in the current month.
Do I have to stop using my spreadsheet?
No, and running both for a month is a sensible way to start. Where the inputs agree the answers should agree, because the math is the same. Where they differ, the difference is usually the inputs, and finding out which is right is worth the month on its own.
What if I am not on Lightspeed?
Then Rebridge cannot help yet, and your spreadsheet is the right tool for now. Rebridge works with Lightspeed Retail R-Series today. Tell us what you run and we will let you know when that changes.
Can I import the plan I already have?
Yes. Monthly budgets import from CSV or Excel with a full preview before anything is written, so you can bring the plan you have rather than retyping it.
What does a spreadsheet still do better?
Three things, honestly. It costs nothing, it works with any point of sale because you key the numbers yourself, and it will do anything you want the moment you want it. A buyer with an unusual method and one location may be better served by the sheet they already have.
See the same numbers without the sheet
The arithmetic, a worked example, and the cost-or-retail choice are all on the open-to-buy page. If you want to compare it against what your own sheet says, that is the place to start.