What Is Float in Construction? Total, Free & Interfering Float

<div class="ge-article-wrapper"><nav class="ge-toc" aria-label="Table of contents"><p class="ge-toc-label">In this article</p><ul class="ge-toc-list"><li><a href="#the-three-types-of-float">The Three Types of Float</a></li><li><a href="#why-float-matters-in-construction">Why Float Matters in Construction</a></li><li><a href="#worked-example-float-consumption-on-a-30m-programme">Worked Example: Float Consumption on a £30M Programme</a></li><li><a href="#float-and-nec4-who-owns-it">Float and NEC4: Who Owns It?</a></li><li><a href="#float-in-evm-the-cpli-connection">Float in EVM: The CPLI Connection</a></li><li><a href="#common-mistakes">Common Mistakes</a></li><li><a href="#frequently-asked-questions">Frequently Asked Questions</a></li></ul></nav><article class="ge-article-body"><p>Float is the amount of time an activity can slip before it delays something else. That "something else" is what separates the three types: total float measures delay to project completion, free float measures delay to the next activity, and interfering float is the difference between the two. If you're running <a href="/en/earned-value">earned value management</a> alongside your programme, float is the schedule health indicator that <a href="/en/earned-value/definitions/schedule-performance-index">SPI</a> alone can't give you.</p><p>SPI tells you whether you're behind schedule in cost terms. Float tells you whether you're about to hit the critical path. They're different questions with different consequences.</p><p>Float is part of the <a href="/en/earned-value/definitions">earned value definitions glossary</a>. For the schedule performance metrics that complement float analysis, see the <a href="/en/earned-value/cpi-spi">CPI and SPI page</a>.</p><h2 id="the-three-types-of-float">The Three Types of Float</h2><p><strong>Total Float (TF):</strong> The time an activity can slip without delaying the project completion date. An activity with 4 weeks of total float can start up to 4 weeks late and the project still finishes on time. When total float hits zero, that activity is on the critical path.</p><p><strong>Free Float (FF):</strong> The time an activity can slip without delaying any successor activity. Free float is always less than or equal to total float. An activity might have 4 weeks of total float but only 1 week of free float, meaning it can slip 1 week without affecting the next activity, but any more than that eats into shared float.</p><p><strong>Interfering Float (IF):</strong> Total float minus free float. It's the float that, when consumed, doesn't delay the successor but does reduce the total float available to downstream activities. It's shared float. Use yours and someone else has less.</p><pre class="ge-ascii-diagram ge-anim"> NETWORK DIAGRAM: TOTAL FLOAT vs FREE FLOAT ════════════════════════════════════════════ CRITICAL PATH (TF = 0): ┌────────┐ ┌────────┐ ┌────────┐ ┌────────┐ │ Found- │───&gt;│ Super- │───&gt;│ Roof │───&gt;│ Ext. │ │ ations │ │ struct │ │ Steel │ │ Clad │ │ 8 wks │ │ 12 wks │ │ 6 wks │ │ 8 wks │ │ TF: 0 │ │ TF: 0 │ │ TF: 0 │ │ TF: 0 │ └────────┘ └────────┘ └────────┘ └────────┘ │ ▲ │ │ ▼ │ ┌────────┐ ┌────────┐ │ │ M&amp;E │───&gt;│ M&amp;E │─────────┘ │ 1st Fix│ │ 2nd Fix│ │ 6 wks │ │ 4 wks │ │ TF: 6 │ │ TF: 6 │ │ FF: 0 │ │ FF: 6 │ │ IF: 6 │ │ IF: 0 │ └────────┘ └────────┘ ─────────────────────────────────────────────── M&amp;E 1st Fix has 6 weeks TF but 0 weeks FF. If it slips 1 day, M&amp;E 2nd Fix is delayed. M&amp;E 2nd Fix has 6 weeks TF AND 6 weeks FF. It can slip 6 weeks without affecting anything. ─────────────────────────────────────────────── </pre><p>Notice the difference. M&amp;E 1st Fix and M&amp;E 2nd Fix have the same total float (6 weeks), but completely different free float. The first fix has no room to slip without impacting its successor. The second fix can slip the full 6 weeks because there's nothing behind it before the critical path converges.</p><p>This distinction matters enormously when you're negotiating NEC4 compensation event time impacts.</p><h2 id="why-float-matters-in-construction">Why Float Matters in Construction</h2><p>Float is your programme's shock absorber. Every project starts with a certain amount of float distributed across non-critical activities. As activities slip, float gets consumed. When it's gone, everything is critical.</p><p>On a well-managed project, float consumption is slow and predictable. On a troubled project, float disappears in the first third of the programme, and the remaining two thirds are spent firefighting.</p><p>I track float consumption rate as a leading indicator. If a £30M programme starts with an average of 6 weeks total float across non-critical paths, and it's consuming float at 1.5 weeks per month, you've got roughly 4 months before every path is critical. That's a deadline for getting the programme back on track. Not completion, but the point beyond which every delay hits the end date.</p><h2 id="worked-example-float-consumption-on-a-30m-programme">Worked Example: Float Consumption on a £30M Programme</h2><span class="ge-worked-label">Worked Example</span><div class="ge-callout ge-anim"><p><strong>Scenario:</strong> A £30M NEC4 Option C highway improvement scheme in the East Midlands. 18-month programme. At contract award, the Accepted Programme shows:</p><ul><li>Critical path duration: 78 weeks</li><li>Average total float on non-critical paths: 6.2 weeks</li><li>Number of non-critical paths: 14</li></ul><p><strong>Month 3 (reporting date: 15 May 2025):</strong></p><ul><li>Average total float remaining: 4.8 weeks</li><li>Float consumed: 1.4 weeks in 3 months (0.47 weeks/month)</li><li>Status: Healthy. At this rate, float lasts 10+ months.</li></ul><p><strong>Month 6 (reporting date: 15 August 2025):</strong></p><ul><li>Average total float remaining: 2.1 weeks</li><li>Float consumed: 2.7 weeks in 3 months (0.9 weeks/month)</li><li>Float consumption has nearly doubled. Three activities that had 4+ weeks of float are now at 0.5 weeks.</li><li>Status: Warning. Something changed between month 3 and month 6.</li></ul><p><strong>The commercial manager investigates:</strong> The ground stabilisation works on the eastbound carriageway hit unexpected contamination. This consumed 3 weeks of float on the drainage and kerbing paths. Two M&amp;E activities were re-sequenced to work around the delay, eating into their free float.</p><p><strong>EVM impact:</strong></p><ul><li><a href="/en/earned-value/definitions/schedule-performance-index">SPI</a> at month 6: 0.94 (6% behind schedule in earned value terms)</li><li>But SPI alone doesn't tell you that 5 of the 14 non-critical paths are now effectively critical</li><li>Float analysis shows the real picture: the programme is one more delay from the critical path shifting</li></ul><p><strong>Month 9:</strong> A compensation event (clause 60.1(12), physical conditions) is notified. The time impact assessment shows 4 weeks of critical delay. The Contractor submits a revised programme showing an extended completion date. The Project Manager assesses the CE using the Accepted Programme and the float position at the dividing date.</p></div><h2 id="float-and-nec4-who-owns-it">Float and NEC4: Who Owns It?</h2><p>This is the argument that won't die.</p><p>NEC4 clause 63.5 states that a compensation event is assessed using the Accepted Programme. (The time impact feeds into the <a href="/en/earned-value/definitions/final-assessment">final assessment</a> calculation at project close.) When assessing the time impact, the PM uses the float shown in the programme at the dividing date. But the contract doesn't explicitly say who owns the float.</p><p>There are two schools of thought:</p><p><strong>"Contractor owns the float"</strong>: Float is the contractor's scheduling efficiency. They created it through good planning. A client delay should consume the client's time, not the contractor's float. The contractor shouldn't be penalised for planning well.</p><p><strong>"Project owns the float" (the more commonly accepted view)</strong>, Float belongs to the project, not either party. A delay of 3 weeks to an activity with 4 weeks of float isn't a compensable delay because it doesn't affect the completion date. The float absorbs it. This is the position most NEC4 adjudicators have taken, though it's not universally settled.</p><p>My view? The contract wording supports the "project owns the float" interpretation, but it's one of those areas where the drafting could be clearer. If you're a contractor who wants to protect your float, show it explicitly in the Accepted Programme and document your planning rationale. If you're a PM, don't assume float is yours to consume. It exists because the contractor planned sensibly.</p><h2 id="float-in-evm-the-cpli-connection">Float in EVM: The CPLI Connection</h2><p>The Critical Path Length Index (CPLI) incorporates float into schedule performance measurement:</p><p><strong>CPLI = (Critical Path Length + Total Float) / Critical Path Length</strong></p><p>A CPLI above 1.0 means the project has float available. Below 1.0 means float is exhausted and the project is forecast to finish late.</p><p>On the highway example at month 6:</p><ul><li>Critical path remaining: 52 weeks</li><li>Total float remaining (on the most constrained non-critical path): 2.1 weeks</li><li>CPLI = (52 + 2.1) / 52 = <strong>1.04</strong></li></ul><p>Barely above 1.0. The programme has almost no buffer left. SPI at 0.94 doesn't convey the same urgency. CPLI at 1.04 does. It says "one more 2-week delay and you're late."</p><p>That's why float analysis and CPLI are better schedule health indicators than SPI alone. SPI measures cost-weighted progress. Float measures actual programme resilience.</p><h2 id="common-mistakes">Common Mistakes</h2><ol><li><strong>Confusing total float with free float.</strong> An activity with 6 weeks of total float sounds comfortable. But if its free float is zero, any slippage immediately delays the next activity. Teams who only look at total float miss critical inter-activity dependencies.</li></ol><ol><li><strong>Not tracking float consumption over time.</strong> A snapshot of float on one reporting date tells you almost nothing. The trend matters. Float that's declining at an increasing rate is a leading indicator of programme failure, often 2-3 months before it shows up in SPI.</li></ol><ol><li><strong>Treating float as a target to consume.</strong> "We've got 4 weeks of float, so we can start that activity 4 weeks late." Technically true. Practically stupid. Float is insurance against unknown delays. Consuming it deliberately on known activities leaves nothing for the surprises that always come.</li></ol><ol><li><strong>Ignoring float ownership in CE assessments.</strong> On NEC4, if the Accepted Programme shows 3 weeks of float on an activity that's delayed by a compensation event, the time assessment should account for that float. Failing to do so can lead to overstatement of the time impact. But equally, assuming all float is "free" to absorb client delays is an oversimplification.</li></ol><div class="ge-product-note ge-anim"><p><strong>How Gather helps.</strong> Gather's AI reads your site diaries daily and maps progress against your cost-loaded programme, giving you accurate earned value data without manual spreadsheet updates. <a href="https://gatherinsights.com/contact">Book a demo</a> to see it working on a live NEC4 project.</p></div><h2 id="frequently-asked-questions">Frequently Asked Questions</h2><h3>What's the difference between float and contingency?</h3><p>Float is time. Contingency is money. Float is the scheduling buffer between non-critical activities and the project completion date. Contingency is the cost buffer built into the budget for unknowns. They serve similar purposes (absorbing risk) but in different dimensions. You can have plenty of float and no cost contingency, or vice versa.</p><h3>Can float be negative?</h3><p>Yes. Negative total float means the activity must finish earlier than currently scheduled to meet the completion date. It indicates the project is already behind, the completion date can't be met without acceleration. Negative float is the programme equivalent of a fire alarm.</p><h3>How often should float be recalculated?</h3><p>Every time the programme is updated, typically monthly, or fortnightly on fast-track projects. Float changes every time an activity starts late, finishes early, or is re-sequenced. A monthly snapshot is the minimum for meaningful trend analysis.</p><h3>Does SPI capture float consumption?</h3><p>Not directly. <a href="/en/earned-value/definitions/schedule-performance-index">SPI</a> measures earned value against <a href="/en/earned-value/definitions/planned-value">planned value</a>, it tells you whether you're earning value at the planned rate. But a project can have an SPI of 0.98 (almost on plan) while float is being consumed rapidly on non-critical paths. By the time SPI drops below 1.0, the float might already be gone. That's why CPLI is the better indicator for schedule health.</p></article></div>