A continuous trench footing for a standard 40×30 foot house needs 4 to 5 cubic yards of concrete. That is 261 bags of 60 lb concrete. No one mixes 261 bags by hand on a job site — trench footings are always poured with a ready-mix truck, and that is exactly how the calculator outputs the result. The cubic yard figure is the number you call the batch plant with. The per-linear-foot unit rate (shown in the table) is what estimators use to quickly scale the cost when a client asks “what if we add a 30-foot interior load-bearing wall footing?” — just multiply the extra footage by the cu yd per linear foot rate.
The IRC R403.1 minimum width check is the feature no other online concrete calculator provides for trench footings. The International Residential Code Table R403.1 sets minimum footing widths based on the number of building stories and the soil bearing capacity. Most homeowners and small contractors use 16 inches as a rule-of-thumb residential minimum, which is a reasonable practical standard for 1-story construction — but a 2-story house on marginal soil technically requires 15 inches per the conservative 1500 psf table, and a 3-story requires 23 inches. Our calculator flags any combination where your entered width falls below the IRC minimum for your selected number of stories, so you know before the concrete inspector shows up at the site.
Building length times building width gives half the perimeter — multiply by 2 for the full perimeter in linear feet. Volume equals linear feet times footing width (feet) times footing thickness (feet), times 1.10 waste. Cubic yards equals total cubic feet divided by 27. Per-linear-foot rate equals total cubic feet divided by linear footage. IRC R403.1 minimum width lookup uses the entered story count and a conservative 1500 psf soil bearing assumption. Width comparison chart shows cubic yards for six standard footing widths (12, 14, 16, 18, 20, 24 inches) at the current length and thickness, with your selected width highlighted in red.
01
Order 10% More Than the Calculator Shows — Then Call 24 Hours Before
Our calculator already includes a 10% waste factor, but trench footings have additional variables that can consume more concrete: soft spots where the trench bottom is lower than planned, over-dug corners, and the unavoidable short loads that remain in the drum at the end of a pour. The standard professional practice is to order 10% more than the calculated volume (which our calculator already accounts for) but also call the batch plant the morning of the pour to confirm the exact yard count and truck timing. Batch plants schedule trucks to arrive at 15 to 20-minute intervals on large pours so the concrete stays workable. If you are pouring a 4-yard footing alone and the first truck arrives before your forms are set, you have a problem. Have your forms, rebar, and stakes fully in place 30 minutes before the truck arrives.
02
Use Rebar and Fiber Reinforcement Together — Not One or the Other
The standard residential trench footing specification calls for two continuous horizontal rebar bars (#4 or #5, depending on load) running the full length of the footing at one-third and two-thirds of the footing thickness. This rebar prevents tensile cracking when the footing carries uneven loads or when soil settles slightly differently under different parts of the structure. In addition to rebar, specifying a fiber-reinforced concrete mix (polypropylene fibers added to the ready-mix at the batch plant) dramatically reduces plastic shrinkage cracking during the first 24 hours of curing. Fiber reinforcement adds approximately $8 to $15 per cubic yard to the mix price and is worth every penny for a footing that will carry a building for the next 50 years. Tell the batch plant dispatcher: 4,000 psi mix with fiber reinforcement.
03
Never Pour a Trench Footing on Disturbed or Frozen Soil
A trench footing transfers the building load to undisturbed native soil — the firm, compacted soil that has not been moved in decades. Footing failures in residential construction almost always trace back to one of two causes: the footing was poured on disturbed fill soil that was not properly compacted, or the footing was poured when the bottom of the trench was frozen and the frozen soil later thawed and settled. Both problems are entirely preventable. When the excavator opens the trench, walk the bottom and verify firm, undisturbed soil before scheduling the pour. If the trench bottom is soft or shows recent fill, call the building inspector before proceeding — the solution is either removing more soil to reach firm native ground or placing compacted crushed stone as a working base. Never pour concrete on frozen soil at any time of year, even in spring when the surface has thawed but sub-surface frost may still be present.
How much concrete does a trench footing need?+
Trench footing volume equals length (ft) times width (ft) times thickness (ft) times 1.10 waste factor. A 140-linear-foot perimeter footing at 16-inch wide by 8-inch thick needs 4.35 cubic yards of ready-mix. A 96-linear-foot garage footing at the same dimensions needs 2.09 cubic yards. The per-linear-foot rate for 16-in wide by 8-in thick footing is 0.031 cubic yards per linear foot — estimators can use this rate to quickly calculate any additional footing runs on the same project.
What are the IRC minimum footing dimensions?+
IRC R403.1 specifies minimum continuous footing widths based on stories and soil bearing. At 1500 psf soil (the conservative assumption): 1-story = 12-in min width; 2-story = 15-in min; 3-story = 23-in min. Minimum thickness is 6 inches per IRC R403.1. The minimum concrete strength is 2,500 psi per IRC R403.2, but most engineers specify 4,000 psi for better durability. Most residential inspectors accept 16-in x 8-in as the standard 1-story footing in normal soil, even though the IRC minimum is 12-in x 6-in. Always confirm with your local building department.
Can I mix bags for a trench footing instead of ordering ready-mix?+
Technically yes, but practically no for anything beyond a very small shed foundation. A standard house footing of 4 cubic yards requires 240 bags of 60 lb concrete. Mixing 240 bags in a rented half-bag mixer at the rate of about 8 bags per load takes 30 mixer loads — a full 12-hour day with two people. Ready-mix delivers the entire volume in one truck in 45 to 90 minutes. The material cost difference is small (ready-mix at $150/yard is $600 vs bags at $6.48/bag is $1,555 for 240 bags). The labor cost difference is enormous. Only consider bags for very small shed footings of less than 1 cubic yard total — everything larger is a ready-mix job.
What concrete strength does a footing need?+
IRC R403.2 requires minimum 2,500 psi concrete for footings not exposed to weathering, and 3,000 psi for footings exposed to freezing and thawing. In practice, most contractors and engineers specify 4,000 psi for all residential footings because the cost difference over 2,500 psi is negligible ($5 to $10 per yard) and the stronger mix provides better resistance to cracking and freeze-thaw deterioration. If you are in a climate with significant freeze-thaw cycling and the footing will be in contact with moist soil (which all footings are), specify 4,000 psi air-entrained concrete with fiber reinforcement from the batch plant.
Does a trench footing need rebar?+
IRC R403.1 does not mandate rebar in residential continuous footings in many low-seismic, non-expansive-soil areas — but this varies significantly by location. High-seismic zones (California, Pacific Northwest, parts of the Mountain West) require continuous rebar per their state-adopted seismic provisions. High-wind areas (Gulf Coast, Atlantic Coast) often require anchor bolts and rebar for structural continuity. Expansive clay soils (common in Texas, Colorado, the mid-South) cause footing movement that rebar resists. The practical standard for most residential footings is two continuous #4 or #5 rebar bars running the length of the footing at 3 inches from the bottom and 3 inches from the top, even when not strictly required by code. It is cheap insurance against cracking.
How wide should a trench footing be?+
The standard practical residential width is 16 inches for 1-story buildings and 20 inches for 2-story buildings. The IRC R403.1 minimums (12-in for 1-story, 15-in for 2-story) represent the absolute code floor but are narrower than most inspectors expect to see. The footing width should be at least twice the stem wall or foundation wall thickness above it — a 8-inch concrete block wall (CMU) sits on a 16-inch footing to give 4 inches of bearing on each side. A 10-inch poured concrete wall sits on a 20-inch footing. These are the practical proportions that have been used in US residential construction for decades and that most local inspectors will approve without requiring additional engineering documentation.
How deep does a trench footing need to go?+
Trench footings must extend below the local frost depth AND below the depth of any fill soil to reach undisturbed native soil. The footing depth varies enormously by US location: 12 to 18 inches in the Deep South; 24 to 36 inches in the mid-Atlantic; 36 to 48 inches in the Midwest; 48 to 60 inches in the Great Lakes states; 60 to 72 inches in the northern plains. The depth in this calculator refers to the footing thickness (not the burial depth) — the footing is typically poured at the bottom of the excavated trench, and the trench depth is determined by frost depth and soil conditions. Confirm the required frost depth with your local building department.
How do I calculate trench footing linear footage for an L-shaped building?+
For an L-shaped, T-shaped, or other irregular foundation, measure the total perimeter by adding up all the exterior wall lengths. For an L-shape with dimensions of 40 x 20 ft (one wing) and 20 x 20 ft (the other wing), the perimeter is 40 + 20 + 20 + 20 + 20 + 40 = 160 linear feet (exterior sides only). Do not simply calculate a rectangular perimeter — trace the actual exterior wall line. If the building also has interior load-bearing wall footings that need concrete, add their total length to the exterior perimeter. Enter the total in the manual linear footage field in our calculator (set both building length and width to 0, or enter the full linear footage in the override field).
What is the difference between a trench footing and a stem wall?+
A trench footing (also called a spread footing or continuous footing) is the horizontal base poured at the bottom of the excavation — it is typically wider than the wall above it and sits in contact with the undisturbed soil. The stem wall is the vertical foundation wall that sits on top of the footing and extends up to the floor level. The two are poured separately in most US residential construction: the footing is poured first and allowed to cure, then forms for the stem wall are set on top of the footing and the stem wall is poured. Some contractors use a monolithic T-footing pour that combines the footing and a short stem wall in a single pour — this saves time but requires more complex forming and is more common in warm climates with shallow frost depths. Our calculator computes the horizontal footing only, not the stem wall.
How long does it take to pour a trench footing?+
A standard residential house footing of 4 to 5 cubic yards takes approximately 2 to 3 hours to place with one ready-mix truck and a crew of 3 to 4 workers. This assumes the trench is already excavated, rebar is in place, and anchor bolts and form stakes are staged and ready. The pour itself (concrete flowing from the truck chute into the trench) takes 30 to 45 minutes. The remaining time is for screeding the top level, setting anchor bolts, final leveling with a hand float, and cleanup. The concrete truck charges waiting time (typically $1 to $2 per minute) after the first 5 to 7 minutes — so have everything ready before the truck arrives.
When can I build on a trench footing after pouring?+
Standard 4,000 psi concrete reaches approximately 60% of design strength in 3 days and 85% at 7 days. Most building inspectors allow stem wall forming and backfilling to begin 24 to 48 hours after the footing pour when normal-temperature curing conditions exist (above 50 degrees Fahrenheit). Actual structural loading — the weight of the building frame — should not be applied until 7 days minimum, and 28 days is preferred before full structural loads are imposed. In cold weather (below 40 degrees), cure times are dramatically extended and the concrete must be protected from freezing for at least 72 hours after placement. Do not pour a footing if a hard freeze is forecast within 48 hours unless you can provide heating and insulating blankets.
Do trench footings need to be level?+
Yes, the top surface of the footing must be level within the tolerances required for the wall system being placed on it. For concrete block (CMU) stem walls, the footing top is typically set dead level because the first course of block must bed level. For poured concrete stem walls, slight variation can be corrected in the stem wall form setting. For slab-on-grade buildings where the footing doubles as the thickened slab edge, level is critical. On sloped sites, the trench footing is stepped down at intervals to maintain a level top surface rather than following the slope of the ground — these are called stepped footings or benched footings. Stepped footings involve horizontal runs at different elevations connected by vertical steps, and require additional concrete volume calculation for each step section.
How do I schedule a ready-mix truck for a small footing pour?+
For small footing pours of 1 to 3 cubic yards, call 2 to 3 local ready-mix plants and ask about their minimum order and short-load fee. Most plants have a minimum order of 1.0 cubic yard with a short-load surcharge of $50 to $150 for anything below their standard minimum (typically 7 to 10 yards). For a 4-yard footing, you are in the range where most plants will deliver a standard residential order without a short-load fee. Call 3 to 5 business days in advance, provide the site address, access information (gate codes, narrow drives), and specify: number of yards, 4,000 psi mix, fiber reinforcement if desired, and your preferred arrival time. The plant will confirm a 2-hour window for the truck arrival.
What is the IRC requirement for concrete footing thickness?+
IRC R403.1 specifies a minimum footing thickness of 6 inches. However, the thickness must also be at least as deep as the footing projects beyond the face of the wall above it — this is the standard bearing design rule that ensures the footing can spread the load. If the footing projects 3 inches on each side of a 10-inch wall (giving a 16-inch footing width), the 3-inch projection requires a minimum 3-inch thickness at the edge. Since the IRC minimum of 6 inches exceeds this, the 6-inch minimum governs in most residential applications. In practice, 8 inches is the standard residential footing thickness that most inspectors expect and that provides an adequate margin above the IRC minimum for typical residential loads and soil conditions.
Do I need a permit for a foundation footing?+
Yes, virtually every US jurisdiction requires a building permit for new construction that includes a concrete foundation footing. The footing inspection is one of the most important inspections in the building process — the inspector visits the site before concrete is poured to verify the trench depth and dimensions, rebar placement, and compliance with the approved plans. Pouring the footing without an inspection means the inspector cannot verify code compliance and may require removal and replacement if problems are discovered. Some jurisdictions require the contractor to call for an inspection at least 24 to 48 hours before the planned pour time. Never schedule the concrete truck before scheduling the footing inspection — the inspector must see the open trench and rebar before concrete covers everything.
How do stepped footings work on a sloped site?+
Stepped footings (also called benched footings) are used when the ground slopes enough that a continuous level footing would either be too deep at the high end or float above grade at the low end. The footing is excavated in horizontal steps that follow the slope, with each step dropping vertically at a transition point. The IRC requires that each horizontal section of the footing be at least 2 feet long, that the vertical step between levels be no taller than the footing thickness, and that the horizontal overlap between the upper and lower footing levels be at least 2 feet. To calculate the concrete for a stepped footing, calculate each horizontal section separately (section length times footing width times footing thickness) and add them all together, plus the concrete for the vertical step transitions. Our calculator handles a single continuous level run — for a stepped footing, run the calculation once per step level and sum the results.