Construction Accounting, Miller Act Surety Bonding & Bid Estimating
Construction Surety Sliding-Scale Bond & Capacity Formula
Calculate Commercial / Public-Works Miller Act Performance & Payment Surety Bond Tiered Sliding-Scale Premium ($), Effective Blended Bond Rate, Working-Capital Aggregate Bonding Capacity ($), and Remaining Unbonded Backlog Headroom.
Stop over-bidding public-works tenders with flat Tier-1 bond rates—use sliding-scale surety brackets!
Under the federal Miller Act and Little Miller Acts, general contractors bidding public infrastructure must post 100% Performance and Payment Surety Bonds! Junior estimators frequently multiply the entire Contract Price by the steep Tier-1 rate (e.g., 1.50% = $15/$1,000)—overstating bond cost by nearly 2x and losing tight tenders! Surety Association Class-B sliding schedules price progressively: First $500,000 at Tier_1_Rate, Next $2,000,000 ($500k to $2.5M) at Tier_2_Rate, and Excess above $2,500,000 at Tier_3_Rate! Simultaneously, surety underwriters cap Aggregate Uncompleted Backlog Capacity at Adjusted_Net_Working_Capital * Surety_Multiplier (typically 10x to 20x CPA-reviewed working capital)!
Verify on a Municipal Water-Plant Bid—Contract_Bid_Price = $10,000,000 (B2), Tier1_First_500k_Rate = 0.015 (1.50% -> $7,500, B3), Tier2_Next_2M_Rate = 0.010 (1.00% -> $20,000, B4), Tier3_Over_2_5M_Rate = 0.007 (0.70% on $7.5M -> $52,500, B5), Adjusted_Net_Working_Capital = $2,500,000 (B6), and Surety_NWC_Multiplier = 15.0x (B7): Sliding-Scale Bond Premium is 7,500 + 20,000 + 52,500 = $80,000 (E2—saving $70,000 vs a naive 1.50% flat calc!), Effective Blended Bond Rate is 0.0080 (0.80% in E3), Aggregate Program Bonding Limit is $37,500,000 (E4), and Remaining Unbonded Backlog Headroom after award is $27,500,000 (E5)!
| Surety Bond & Capacity Metric ($10M Bid, 1.5%/1.0%/0.7% Tiers, $2.5M NWC @ 15x) | Formula | Verified Output |
|---|---|---|
| Sliding-Scale Bond Premium ($) | MIN(C,500k)*r1 + MIN(MAX(0,C-500k),2M)*r2 + MAX(0,C-2.5M)*r3 | $80,000 ($6,000 @ $400k Bid) |
| Effective Blended Bond Rate | Sliding_Scale_Premium / Contract_Price | 0.0080 (0.0150 @ $400k Bid) |
| Aggregate Bonding Capacity ($) | Adjusted_Net_Working_Capital * Multiplier | $37,500,000 |
| Remaining Backlog Headroom ($) | Aggregate_Capacity - Contract_Bid_Price | $27,500,000 ($37.1M @ $400k Bid) |
Tiered Sliding-Scale Surety Bond Premium Formula
Validate Contract_Price > 0, descending bracket rates 0 < Tier_3 <= Tier_2 <= Tier_1 < 0.05, NWC > 0, and Multiplier > 0!
=LET(ct_p, B2, r_1, B3, r_2, B4, r_3, B5, nw_c, B6, ml_s, B7, IF(OR(ct_p <= 0, r_3 <= 0, r_2 < r_3, r_1 < r_2, r_1 >= 0.05, nw_c <= 0, ml_s <= 0), "Invalid Surety Schedule", LET(t_1, MIN(ct_p, 500000) * r_1, t_2, MAX(0, MIN(ct_p - 500000, 2000000)) * r_2, t_3, MAX(0, ct_p - 2500000) * r_3, ROUND(t_1 + t_2 + t_3, 2))))Small-Project Tier-1 Boundary Check (Set Contract Bid = $400,000)
Test a $400,000 culvert repair bid (B2 = 400000, entirely inside the first $500k bracket): Tier-2 and Tier-3 terms evaluate cleanly to 0, giving $6,000 premium (E2) at the exact 0.0150 Tier-1 rate (E3)!
=IF(ISTEXT(E2), E2, ROUND(B6 * B7 - B2, 2))Try the example
Use the original values or change them to test your understanding.