Stocks and flows: why your pipeline is a bathtub

Your pipeline is a bathtub. The water level — the value of live opportunities sitting in your CRM today — is a stock; the tap filling it (new qualified conversations) and the drain emptying it (deals won, lost, or gone quiet) are flows. The level can only change when the flows change, yet most firms manage the level and ignore the taps.

The stock-and-flow distinction comes from Donella Meadows' Thinking in Systems, and it is the single most practical idea in that book for anyone running a service firm. It sits alongside loops, delays, and leverage points in the toolkit I have set out in A Systems-Thinking Guide for Founders; this piece is the deep dive on one instrument, and I have pulled out the rest of what the book offers operators in Thinking in Systems: what founders should steal.

What is a stock, and what is a flow?

A stock is an accumulation you could measure at an instant: cash in the bank, live opportunities, unanswered enquiries, your own energy on a Monday. A flow is a rate that changes a stock over time: invoices paid per month, new conversations opened per week, enquiries answered per day.

Two rules follow, and they are close to physics:

• Stocks change only through their flows. There is no third way to raise the bath. If pipeline value is falling, then either the inflow of new opportunities has slowed or the outflow has quickened. One of those two things, always.
• Stocks lag their flows. Turn the tap off and the bath stays comfortably full for a while. This is why a founder who stops selling in a busy March feels no pain until June — the stock masks the flow.

Why do founders misread the bathtub?

Because the stock is what dashboards show and what feels real. "We have £180k in pipeline" is a satisfying sentence. But a stock figure without its flows is a photograph of a bath with no information about the taps — the same £180k is healthy if inflow is steady and alarming if the tap was turned off six weeks ago.

The misreading has a predictable shape. Revenue is fine, so selling feels optional; the inflow quietly drops; the stock drains invisibly for a couple of months; then revenue dips and everyone scrambles at the tap. The scramble works, eventually, and the cycle repeats. Firms call this "feast and famine" as though it were weather. It is plumbing.

How do you run a pipeline as a bathtub?

Here is the mechanism, step by step:

1. Measure the stock weekly: count and value of live opportunities. One number, same day each week.
2. Measure the inflow weekly: new qualified conversations opened. When inflow is measured on its own line, then a slow week is visible immediately instead of two months later through revenue.
3. Measure the outflow and split it: won, lost, expired. When "expired" dominates, then the problem is follow-up discipline, not demand.
4. Set a floor on the inflow, not the stock. When the rule is "eight new conversations a week, regardless of how busy delivery is", then the tap keeps running precisely in the weeks the old habit would have shut it.
5. When the stock rises faster than delivery capacity, then throttle deliberately — raise prices or tighten qualification — rather than letting response times do the throttling for you.

None of this requires software beyond a spreadsheet. It requires deciding that flows, not levels, are what you manage.

Which flow should you fix first?

Almost always the inflow, because it is the flow most firms leave to chance. Referrals and repeat work fill the bath without anyone watching the tap, which works until it doesn't. Building a deliberate inflow — a list, a sequence, a daily sending rhythm — is exactly what an outbound system is: a tap you own and can open on schedule. It is the logic behind our Outbound Engine, and the right shape of that tap varies usefully by sub-vertical, which I have mapped in Growth Systems by Industry.

The outflow matters too, but differently: deals that die of neglect are water lost through a crack rather than the drain, and the fix is process, not more selling.

One caution. Once you can see stocks and flows, the temptation is to optimise every flow at once. Resist it — some taps move the level far more than others, and knowing which is the subject of Leverage points: where small changes move revenue.

The bathtub is not a metaphor you outgrow. Cash, pipeline, capacity, and reputation are all stocks, all governed by their flows, all lagging. The firms that feel steady are not luckier; they just watch the taps.

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