How do I get the uplift load on a post base?

Uplift comes from wind or seismic load cases that try to lift the structure off its footing. It is the net upward force after subtracting dead load. For a freestanding pergola or sign, wind on the roof or panel multiplied by the tributary area gives the uplift at the base.

What is a reasonable safety factor?

Published allowable loads already include a safety factor against the connector's ultimate capacity, so a utilization at or below 1.0 means the connector is adequate as listed. The tool also reports the ratio of allowable to demand so you can see remaining margin.

Does the anchor or the base govern?

Whichever has the lower allowable load governs. A strong post base bolted with an undersized concrete anchor fails at the anchor. The tool compares both the base allowable and the concrete anchor allowable to your demand and reports the lower of the two.

Are these the exact manufacturer values?

No. The tool uses representative allowable loads for common base series to support planning and comparison. Always confirm against the current manufacturer catalog for your exact model, wood species, anchor, and concrete strength before building.

What about the concrete edge distance?

Cast-in and post-installed anchors lose capacity near a concrete edge or when spaced closely. This tool checks the headline anchor allowable only. For anchors near an edge or in thin slabs, reduce capacity per the anchor manufacturer's edge-distance and spacing tables.

Post Base Anchor Sizing Calculator

A post base transfers uplift and lateral loads from a wood or steel post into a concrete footing through the connector and its anchor bolt. This tool compares your design uplift and shear demands to representative published allowable loads for common base series and the concrete anchor, then reports utilization and the governing element.

How it works

Connectors are sized by comparing demand to a listed allowable load that already embeds a safety factor:

utilization (uplift) = uplift demand / allowable uplift
utilization (shear)  = shear demand  / allowable shear
governing allowable  = min(base allowable, anchor allowable)
adequate when both utilizations <= 1.0

The smaller of the base’s published allowable and the concrete anchor’s allowable controls, because a load path is only as strong as its weakest link. The tool reports the ratio of allowable to demand so remaining margin is visible.

Example and tips

A 4x4 deck post with 900 lb of wind uplift and 350 lb of shear on a mid-size base rated near 1,800 lb uplift and 700 lb shear, anchored by a bolt good for 1,200 lb, is governed by the anchor at 1,200 lb — a utilization of 0.75 in uplift, which passes. When loads are close to the limit, step up to the next base size, increase the anchor embedment or diameter, and keep anchors well away from slab edges where concrete breakout capacity drops sharply.